466 'Journal of Comparative Neurology and Psychology. 



The form of the ventricle is shown in Fig. 2, and it is necessary 

 only to point ont that quite distinct lateral ventricles are present, 

 that they extend into the olfactory bulbs and that they are connected 

 with the median ventricle by the interventricular foramina. A tri- 

 angular prolongation of the median ventricle projects a short distance 

 in front of the interventricular foramina to meet the vascular canal 

 mentioned above. This is the recessus neuroporicus. 



The solution of the problems of the anterior end of the brain and 

 of the boundary between diencephalon and telencephalon requires the 

 study of the development. Although all the essential facts were first 

 acquired from the study of amphibian embryos (see below), it was 

 thought best to extend the study to other vertebrates, and as the sela- 

 chians present the most primitive conditions the description of certain 

 important processes hitherto overlooked will be given first as they 

 are seen in embryos of Squalus acanthias. 



For the material for this study I am deeply indebted to Dr. H. V. 

 Keal. AVlien I wrote to Dr. Xeal of my findings in Amblystoma 

 and of the desirability of a control study upon selachian embryos 

 he generously sent me from his magnificent collection of mounted 

 preparations of Squalus embryos, the specimens representing the 

 stages from B to M of Balfour's notation. These preparations in- 

 clude whole mounts and sections in all three planes prepared by vari- 

 ous stains. These are the preparations upon which was based deal's 

 very valuable paper of 1898 on the segTuentation of the nervous 

 system. My attention has been directed chiefly to stages from F (15 

 somites) to K and M. It has been a great pleasure to verify the clear 

 and accurate descriptions given by Neal of these embryos. I have 

 studied the brain, cranial ganglia, the mesectoderm derived from the 

 neural crest, the mesodermic somites, the branchial clefts and arches, 

 and have given esj^ecial care to the region about the forebrain and the 

 premandibular and anterior head cavities. I have made plate recon- 

 structions from frontal and sagittal sections of several stages and have 

 had numerous camera drawings made to illustrate points not shown 

 in the models. 



I wish to comment briefly, on the basis of these preparations, upon 

 certain problems that have been under discussion in past years which 

 have some bearing on the special problems being considered here. 



