LATER DEVELOPMENT OF CHICK AND RABBIT 429 

 part of the succeeding neuromeres. Its walls thicken slowly and 



ao 



A 



vt 



ao 



vt 



FIG. 150.- Sagittal sections through the head of the chick. In A 

 the heart is shown in optical section. From Kellicott. 



A, of an embryo with twenty-two or -three pairs of somites (about forty-four hours) ; B, of an 

 embryo with thirty-nine pairs of somites (end of the fourth day), a., auricle ; am., amnion ; ao., 

 dorsal aorta ; ba., bulbus arteriosus ; c/., cranial flexure ; D., diencephalon ; dv., ductus venosus; 

 <?., epiphysis ; h., hypophysis ; '., infuudibulum ; ip., anterior intestinal portal ; is., isthmus ; 

 /., rudiment of lung ; li., liver ; m., mandibular arch ; Ms., mesencephalon ; Mt., metencephalon 

 My., myelencephalon ; n., notochord (degenerating) ; o., oral membrane (oral plate) ; oe., oeso- 

 phagus ; p., pharynx ; P., parencephalon ; r., optic recess ; S., Seessel's pocket (preoral gut) ; 

 st., stomach ; sv., sinus venosus ; Sy., synencephalon ; T., telencephalon ; t h., rudiment of thyroid 

 body ; tp., tuberculum posterius ; v., ventricle ; vt., velum transversum. 



steadily, and finally its roof becomes transformed into the cerebellum 

 and its floor into the pons Varolii. 



