182 BULLETIN OF THE 



From Figure 17, Plate II., it will be seen that the primitive brain of 

 the chick does not correspond with the above description. As the result 

 of my studies, I find that primarily the chick's brain consists of a suc- 

 cession of neural vesicles, from the first of which the three anterior 

 divisions of the adult brain are developed, namely, Prosencephalon, 

 Thalamencephalon, and Mesencephalon. Soon after the formation of 

 the first cerebral vesicle, the optic lobes appear as lateral outgrowths 

 from it, after which, for some time, the growth of this vesicle is not 

 relatively greater than the growth of the following vesicles, or than that 

 of the entire embryo. But with the first appearance of the cranial 

 flexure, the primary fore-brain begins to develop very rapidly, soon a 

 constriction marks off the mid-brain (Fig. 15), after which the fore- 

 brain grows out as an anterior vesicle, so that by the time the head is 

 completely bent, the primary fore-brain is represented by three large 

 vesicles, which open widely into each other (Fig. 14). The develop- 

 ment of these three vesicles is wonderfully rapid, and since it is exactly 

 coincident with the cranial flexure one may well connect the two events 

 causally, affirming the cranial flexure to result from the very rapid 

 development (increase of surface) of the dorsal and lateral walls of 

 the first cerebral vesicle. 



The second primary vesicle develops into the cerebellum, and the 

 successive vesicles, including the neural swellings between the first five 

 protovertebrte, take part in the formation of the adult medulla oblon- 

 gata. Thus, either the development of the brain in the chick forms a 

 marked exception to the usual development of the vertebrate brain, or 

 the nomenclature of the three primary cerebral vesicles is inaccurate. 

 The prevalent impression, that the brain consists primarily of but 

 three vesicles, may possibly arise from the fact that the ectoderm does 

 not exactly follow the neural conformation. The surface of the head 

 at an early stage presents three main divisions, marked by slight con- 

 strictions, but these are quite independent of the neural tract. The 

 difference between the contour of the head and that of the brain will 

 be manifest if the chick be first examined as an opaque object, and then 

 by transmitted light. In the former case, the superficial outlines of the 

 head are most apparent ; in the latter, the medullary walls come into 

 strong relief. 



Although I agree with Orr and Beraneck in regard to the number 

 and appearance of the neuromeres, and although I also find the ulti- 

 mate relation of the cranial nerves to these folds in salmon and chick 

 to be the same as that which they describe for lizard and chick, I do not 



