48 THE STUDY OF CHICK EMBRYOS 



This constitutes the vitelline circulation, and through it the embryo 

 receives nutriment from the yolk for its future development. 



In studying transverse sections of the embryo it is not sufficient 

 merely to identify the structures seen. The student should determine 

 also the exact level of each section with respect to Figs: 40, 41 and 42, 

 and trace the organs from section to section in the series. It is important 

 to remember that the transverse sections figured and described in this 

 manual (except those of the fifty-hour chick) are all drawn viewed from 

 the cephalic surface; hence the right side of the embryo is at the reader's 

 left. 



TRANSVERSE SECTIONS 



Section through the Fore-brain and Optic Vesicles (Fig. 43). The optic stalks con- 

 nect the optic vesicles laterally with the ventral portion of the fore-brain. Dorsally, the 

 section passes through the mid-brain, due to the somewhat ventrally flexed head (cf. Fig. 



Mid-brain 



"Mesenchyme 

 Optic vesicle 



^^ -Lens anlage 



Ccelom^ 



Somatopleure^ __ 



-Splanchnopleure 



FIG. 43. Transverse section through the fore-brain of a thirty-eight-hour chick embryo. X 75. 



42). We have alluded to the thickening of the lens placode. Note that there is now a 

 considerable amount of mesenchyme between the ectoderm and the neural tube. Layers 

 of mesoderm are present in the underlying blastoderm. 



Section through the Pharyngeal Membrane and Mid-brain (Fig. 44). In the mid- 

 ventral line the thickened ectoderm bends up into contact with the entoderm of the rounded 

 pharynx of the fore-gut. At this point the oral opening will break through. On either 

 side of the pharynx a pair of large vessels is seen; the ventral pair are the ventral aorta. 

 Two sections cephalad their cavities open into those of the dorsal pair, the descending 

 aortae. The section is thus just caudad of the point where the ventral aortae bend dorsad 

 and caudad to form the descending aortae. The section passes through the caudal end of 

 the mesencephalon which is here thick walled with an oval cavity. Note the large 

 amount of undifferentiated mesenchyme in the section. The structure of the blastoderm 

 is complicated by the presence of collapsed blood vessels. 



