STRUCTURE OF THIRTY-THREE HOUR CHICKS 



63 



prosencephalon known because of its peculiar shape as the 

 infundibulum (Figs. 23 and 24). The infundibular region is 

 the site of important changes later in development. At this 

 stage, conditions are not sufficiently advanced to warrant more 

 than calling attention to its origin from, and relations to, the 

 prosencephalon, and to the antei^ior end of the notochord as 

 shown in the figures referred to. 



anterior neuropore 



optic vesicle 



prosencephalon 



infundibulum 



bulbo-conus arteriosus 

 cut epi-myocardium 



ventricular region 

 atrial region 



anterior intestinal portal 



ventral aortic roots 



cut ectoderm 



dorsal aortae 



sinus venosus 



lateral mesoderm 



om phalomesenteric 



cut splanchnopleure 



Fig. 23. — Diagrammatic ventral view of dissection of a 35-hour chick embryo. 

 {Modified from Prentiss.) The splanchnopleure of the yolk-sac cephalic to the 

 anterior intestinal portal, the ectoderm of the ventral surface of the head, and 

 the mesoderm of the pericardial region, have been removed to show the under- 

 lying structures. Figure 24 should be referred to for the relations of the peri- 

 cardial mesoderm. 



In chicks of about 2>^ hours indications of the impending 

 division of the three primary vesicles to form the five regions 

 characteristic of the adult brain are already beginning to ap- 

 pear. In the estabhshment of the five-vesicle condition of 

 the brain, the prosencephalon is subdivided to form the 



