and lying side by side, and the corpus spongiosum, surrounding the urethra and lying below and between the corpora cavernosa. The two latter are surrounded by a dense connective-tissue sheath, the tunica albuginea. These erectile bodies are surrounded by a thin layer of skin, containing no adipose tissue and no hair-follicles. The corpus spongiosum is enlarged anteriorly to form the glans penis.

The principal substance of the erectile bodies is the so-called erectile tissue: septa and trabeculæ, consisting of connective tissue, elastic fibers, and smooth muscle-cells inclosing a system of communicating spaces. These latter may be regarded as venous sinuses, the walls of which, lined by endothelial cells, are in apposition to the erectile tissue. Under certain conditions the venous sinuses are distended with blood, but normally they are in a collapsed state and form fissures which simulate the clefts found in ordinary connective tissue. In other words, there is here such an arrangement of the blood-vessels within the erectile tissue that the circulation may be carried on with or without the aid of the cavernous spaces. The arteries of the corpora cavernosa possess an especially well-developed musculature. They ramify throughout the trabeculæ and septa of the erectile tissue and break up within the septa into a coarsely meshed plexus of capillaries. A few of these arteries empty directly into the cavernous spaces. On the other hand, the arteries give off a rich and narrow-meshed capillary network immediately beneath the tunica albuginea. This is in communication with a deeper and denser venous network, which, in turn, gradually empties into the venous sinuses. Aside from these there are anastomoses between the arterial and venous capillaries, which later communicate with the venous network just mentioned. The blood current, regulated as it thus is, may pass either through the capillaries alone, or may divide and flow through both these and the venous sinuses. These conditions explain both the erectile and quiescent state of the penis. The relations are somewhat different in the corpus spongiosum urethræ and in the glans penis.

The epithelium of the urethra varies in the several regions. The prostatic portion possesses an epithelium similar to that of the bladder. In the membranous portion, the epithelium may be similar to that found in the prostatic portion, but more often presents the appearance of a pseudostratified epithelium with two or three layers of nuclei. The cavernous region is lined by pseudostratified epithelium, except in the fossa navicularis, where a stratified squamous epithelium is found. Between the fibro-elastic mucosa and the epithelium there is a basement membrane. There occur in the urethra, beginning with the membranous portion, irregularly scattered epithelial sacculations of different shapes. Some of these show alveolar branching, and are then known as the glands of Littré.

The submucosa of the cavernous portion of the urethra, which

contains nonstriated muscle-tissue arranged circularly, is richly supplied with veins, and contains pronounced plexuses communicating with cavernous sinuses, which correspond in general to those of the corpora cavernosa penis.

In the glans penis the cavernous spaces are small and of more regular shape than in the corpora cavernosa. The glans is covered by a layer of stratified squamous epithelium, often possessing a thin stratum corneum (see Skin).

Near the corona of the glans penis there are now and then found small sebaceous glands (see Hair), known as glands of Tyson. The prepuce is a duplication of the skin, the inner surface presenting the appearance of a mucous membrane.

The nerves terminating in the glans penis have recently been studied by Dogiel, who made use of the methylene-blue method in his investigation. He finds Meissner's corpuscles in the connectivetissue papillæ under the epithelium, Krause's spheric end-bulbs somewhat deeper in the connective tissue, and the genital corpuscles situated still deeper (see Sensory Nerve-endings). In the epithelium are found free sensory nerve-endings. Pacinian corpuscles have also been found in this region.

4. SPERMATOGENESIS.

In order that the student may obtain an understanding of the complicated process of spermatogenesis we shall give a description of it as it occurs in salamandra maculosa, which of all vertebrate animals presents the phenomena in their simplest and best known form. The student should understand, however, that many of the details here described have not been observed in the testes of mammalia; and, since the spermatozoa of many of the mammalia are of simpler structure than those of the salamander, the development of the spermatozoa of the former is consequently simpler. It should also be noticed that the general structure of the testes of the salamander differs in some respects from that of the testes of mammalia, as given in the preceding pages.

At first the seminiferous tubules consist of solid cellular cords, and it is only during active production of spermatozoa that a central lumen is formed, in which the spermatosomes then lie. The cells which compose these solid cords may be early differentiated into two classes-those of the one class being directly concerned in the production of the spermatosomes; those of the other appearing to have a more passive rôle. The cells of the first class—the spermatogonia, or primitive seminal cells-undergo a process of division accompanied by an increase in size. In this way they soon commence to press upon the cells of the second class-the follicular or sustentacular cells. The result is that the nuclei of the latter are forced more or less toward the wall of the seminal tubule, while their protoplasm is so indented by the adjacent spermatogonia that the cells

assume a flattened cylindric shape presenting indentations and processes on all sides. In this stage the spermatogonia have a radiate arrangement and entirely surround the elongated sustentacular cells. At present three periods are distinguished in the development of the male sexual cells (spermatosomes) from the spermatogonia. The first period embraces a repeated mitotic division of the spermatogonia—the period of proliferation. In the second, the spermatogonia, which have naturally become smaller from repeated division, begin to increase in size-the period of growth. The third is characterized by a modified double mitotic division without intervening period of rest, and results in the matured spermatozoa-the period of maturation, figure 283. During the third period, a very important and significant process takes place the

Fig. 283.-Schematic diagram of spermatogenesis as it occurs in ascaris (after Boveri). ("Ergebn. d. Anat. u. Entw.," Bd. 1.)

reduction in the number of chromosomes, so that in the spermatids, the chromosomes are reduced to half the number present in a somatic cell of the same animal. The manner in which this reduction in the number of chromosomes takes place will be described as it occurs in salamandra maculosa.

After the cells composing the last generation of spermatogonia have attained a certain size (period of growth), they undergo karyokinetic division. First, the usual skein or spirem is formed, but instead of dividing into twenty-four chromosomes, as in the somatic cell, the filament of the skein segments into only twelve loops. The cell thus provided with twelve chromosomes now enters upon the period of maturation, and is known as a spermatocyte of the first order, or a "mother cell" (O. Hert

It

wig, 90). The division of these cells is heterotypic (vid. p. 64); the chromosomes split longitudinally and in such a way that the division begins at the crown of the loops, extending gradually toward their free ends. In this case the daughter chromosomes remain for some time in contact, so that the metakinetic figure resembles a barrel in shape. Finally, the daughter chromosomes separate and wander toward the poles. As soon as the daughter stars (diaster) are developed, the number of chromosomes is again doubled by a process of longitudinal division. The spermatocyte of the first order thus divides into two spermatocytes of the second order, or daughter cells (O. Hertwig, 90). The nuclei of the daughter cells now contain twenty-four chromosomes, as is the case in the somatic cell, and, without undergoing longitudinal splitting, the daughter chromosomes are distributed to the two nuclei of the spermatids. In other words, the latter contain only twelve chromosomes. The spermatozoa are formed from the spermatids by a rearrangement of the constituent elements of these cells. may thus be said that even in the stage of the segmenting skein in the mother cells, the spermatocytes of the first degree contain twice as many chromosomes as a somatic cell, a condition which is first clearly seen in the stage of the diaster (here only an apparent duplication in the diaster stage). As a result, there is, first, a decrease in the double number of chromosomes found in the spermatocytes of the second degree to the normal number; second, a decrease in the number of chromosomes in the spermatocytes of the third degree (spermatids) to one-half the number present in a somatic cell, a condition probably due to the fact that here there is no stage of rest nor longitudinal splitting of the chromosomes. This is the general process in heterotypic division. Besides the heterotypic form, there occurs in the division of the spermatocytes another (homeotypic) form of karyokinetic cell-division. This differs from the heterotypic in the shortness of the chromosomes, the absence of the barrel phase, the late disappearance of the aster, and the absence of duplication in the chromosomes of the diaster. According to Meves (96), the spermatocytes of the first degree undergo heterotypic, those of the second degree, homeotypic division.

The spermatids develop into the spermatozoa, beginning immediately after the close of the second division of maturation. This process has been fully described for salamandra maculosa by Hermann, Flemming, Benda, and others, but need not engage our attention at this point beyond the statement that the chromatin of the nuclei of the spermatids develops into the heads of the spermatozoa, while the remaining structures are developed from the protoplasm. "The mature spermatozoon of the salamander represents a completely metamorphosed cell; in the course of its development no portion of the original cell is cast off" (Meves, 97).

Spermatogenesis in mammalia may be compared to the foregoing

process, with the exception that here the different stages are seen side by side in the seminiferous tubule and without any apparent sequence, making the successive stages more difficult to demonstrate. The various generations of cells form columns, and are arranged in such a manner that the younger are found near the lumen and the older close to the wall of the tubule. (Figs. 284 and

Fig. 284. Schematic diagram of section through convoluted seminiferous tubule of mammal, showing the development of the spermatosomes. The number of chromosomes is not shown in the various generations of the spermatogenic cells. The progressive development of the spermatogenic elements is illustrated in the eight sectors of the circle: a, Young sustentacular cell; b, spermatogonium; c, spermatocyte; d, spermatid. In 1, 2, 3, and 4 the spermatids rest on the enlarged sustentacular cell in the center of the sector; on both sides of the sustentacular cells are the spermatogenic or mother cells in mitosis. In the sectors 5, 6, 7, and 8 spermatozoa are seen in advanced stages resting on the sustentacular cells, with new generations of spermatids on each side. [From Rauber (after Brown) with changes (after Hermann).]

285.) These columns are separated from each other by high sustentacular cells, or Sertoli's cells or columns. The metamorphosis of the cells into spermatids and spermatosomes is accomplished by the changing of the cells bordering upon the lumen and then of those in the deeper layers, etc., into spermatids and then into spermatosomes. During this process the spermatids arrange