Notwithstanding the publication of a great mass of descriptive detail regarding the structure and morphology of the forebrains of lower vertebrates, it is very difficult to form a clear picture of the fundamental morphological features of the vertebrate cerebral hemisphere. This wealth of observation has stubbornly resisted correlation and the morphological fruits of these arduous labors have until very recently, it must be confessed, been disappointingly meager. We have, however, now reached a point where effective correlation has begun to take form and within the bewildering complexity of detail characteristic of individual species it is possible to see a common morphological pattern which is sur THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGY, VOL. 20, No. 5. prisingly constant and very simple. In this contribution I have brought together certain features of embryonic and adult brains of amphibians and reptiles which illustrate the fundamental simplicity of this pattern. Johnston has recently presented strong evidence that the telencephalon must be regarded as the terminal segment of the neural tube, a view confirming (with some modifications) the original teachings of His as opposed to the usage in the BNA tables In the latter the diencephalon is regarded as extending to the extreme rostral end of the primary neural tube, thus comprising the whole of the unpaired ventricle of this part of the brain and its lateral walls, including the lamina terminalis, while the telencephalon is regarded as limited to the secondarily evaginated parts of the neural tube termed the cerebral hemispheres, viz., the lateral ventricles and their massive walls. The usage of His and Johnston implies that the rostral part of the third ventricle, bounded behind by the velum transversum above and the chiasma-ridge (Johnston) below, and its walls are to be regarded as telencephalon. medium, while the evaginated hemispheres constitute the telencephalon laterale. Johnston has further shown that in lower vertebrates there has been a progressive tendency as we pass up the developmental series (both ontogenetic and phylogenetic) for more and more of the telencephalon medium to be evaginated through the interventricular foramina into the hemispheres. These considerations have an obvious bearing on the problem of the relation of the cerebral cortex to the primordial tissues from which it has been differentiated. With a view to the contribution of further data for the solution of this problem, I have examined the embryonic and adult brains of a series of types of lower vertebrates, the first results of which are presented in this paper. I shall discuss the brains of fishes only incidentally and devote my attention chiefly to the amphibians and reptiles, whose cerebral hemispheres have evaginated so far from the primordial neural tube as to present a form approximating more closely to the mammalian conditions and readily leading up to them. Our immediate problem, then, is the relations of the first recognizable primordia of the cerebral cortex to the other elements of the evaginated cerebral hemisphere and of all of these structures to the more ancient tissues of the telencephalon medium and diencephalon. My indebtedness to the published works of Johnston will be evident to the reader throughout this paper. I have received still greater assistance from many extended conferences with Professor Johnston, in which he has freely shared with me his unpublished observations and stimulatng suggestions. The full extent of this obligation it is not necessary, nor indeed possible, for me to indicate here. It should, moreover, be added that, while many parts of this discussion have been greatly influenced and I trust improved by these conferences, the responsibility for the morphological views here expressed is wholly my own. AMPHIBIA I have studied an extensive series of sections of larval and adult Amblystoma, Necturus and various species of frogs, prepared by different methods, including the silver methods of Golgi and Ramón y Cajal, the method of Weigert, a toluidin blue modification of Nissl's method and various general embryological methods. Most of this material, except the larval Necturus which I studied through the courtesy of Professor Minot in the Harvard Embryological Collection, was prepared by Mr. P. S. McKibben of the Department of Anatomy, University of Chicago, to whose kindness and skill I am greatly indebted. I have also examined a series of cross sections through the head of Petromyzon (Ichthyomyzon concolor) prepared and kindly loaned to me by Dr. Charles Brookover. In the Amphibia the wall of the cerebral hemisphere is naturally divided into five parts. Not to prejudice the morphological significance of these parts at the start, I shall call them simply olfactory bulb, ventro-medial, ventro-lateral, dorso-lateral and dorso-medial parts. They are especially distinct in the adult frog and are termed by Gaupp respectively lobus olfactorius, |