44 



THE STUDY OF CHICK EMBRYOS 



adjacent to the posterior end of the heart and later connects with it. Another pair, 

 the vitelline arteries, is developed in continuation with the aortae of the embryo. The 

 vessels of the vascular area thus appear before those of the embryo have developed ; they 

 probably arise from the splanchnic mesoderm, and, both arteries and veins, are composed 

 of a simple endothelial wall. As the ccelom develops in the region of the vascular area 

 of the embryo soon after the differentiation of the angioblast, the anlages of the blood 

 vessels are formed only in the splanchnic layer. (For the development of the heart and 

 blood vessels see Chapter IX.) 



Ectoderm 



Neural tube 



Mesenchym 



Entoderm of 

 proamnion 



Descending aorta 



-^z Fore-gut 



Ectoderm of proamnion 



\\ 



FIG. 38. Transverse section through the pharyngeal membrane of a twenty-five-hour chick 



embryo. X 90. 



Section through the Pharyngeal Membrane (Fig. 38). This section passes through 

 the head fold and shows the head free from the underlying blastoderm (cf. Fig. 42). Sec- 

 tions a little caudad in the series prove that this is accomplished by folds of somatopleure. 

 These bend in from the front and sides, fuse, and the head is progressively 'pinched off' 

 from the blastoderm (see pp. 80-81). The ectoderm surrounds the head, and near the 

 mid-ventral line it is bent dorsad, is somewhat thickened, and comes in contact with the 

 thick entoderm of the pharynx. The area of contact between ectoderm and pharyngeal 

 entoderm forms the pharyngeal plate, or membrane. Later, this membrane breaks through 



Proamnion 



FIG. 39. Transverse section through the fore-brain and optic vesicles of a twenty-five-hour 



chick embryo. X 90. 



and thus the oral cavity arises. The expanded neural tube is closed in this region and 

 forms the middle brain vesicle, or mid-brain. The descending aortae appear as small ves- 

 sels dorsal to the lateral folds of the pharynx. The blastoderm in the region beneath the 

 'head is composed of ectoderm and entoderm only. This is the proamniotic area. Laterad 

 may be seen the layers of the mesoderm. 



Section through the Fore-brain and Optic Vesicle (Fig. 39). The neural tube is open 

 here and constitutes the first brain vesicle, or fore-brain. The opening is the anterior 

 neuropore. The ectoderm is composed of two or three layers of nuclei and is continuous with 



