EMBRYO OF SEVENTEEN SEGMENTS 



43 



or membrane. Later, this membrane breaks through 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 vessels dorsal to the lateral folds of the pharynx. The 

 blastoderm in th e region bene ath the head is composed of e ctoder m and entoderm jmly^ 

 This is the proamnwHr, are a. Laterad may be seen the layers of the mesoderm. 



Ectoderm 



Mesenchyma 



Notochord 



Pharyngeal 

 membrane 



Entoderm of ' ^■r'"""™"^^;^^'^. 

 proamnion '^.^ --^•— 



Neural lube 



jr 



,; . ^ fj — i— — Fore-gut 



Descending aorta 







'&-— Ectoderm of proamnion 



Fig. 38. — Transverse section through the pharyngeal membrane of a twenty-five-hour chick embryo. 



X 90. 



Transverse 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 neurop ore. The ectoderm is composed of two or three layers of nuclei and is con- 

 tinuous with the much thicker wall of the fore-brain. The lateral expansions of the fore- 



N euro pore 



■Neural tube 



Fig. 39. — Transverse section through the fore-brain and optic vesicles of a twenty-five-hour chick. X 90. 



brain are the optic vesicles, which eventually give rise to the retina of the eye. The two ecto- 

 dermal layers are in contact with each other except in the mid-ventral region, where the 

 mesenchyma is beginning to penetrate between and separate them. The proamnion consists 

 merely of a layer of ectoderm and of entoderm. 



CmCK EMBRYO OF SEVENTEEN PRIMITIVE SEGMENTS (THIRTY-EIGHT HOURS) 



The long axis of this embryo is nearly straight (Fig. 40), the area pellucida 

 is dumb-bell shaped and the vascular network is well differentiated throughout 

 the area opaca. The tubular heart is bent to the embryo's right, and opposite its 

 posterior end the vascular network converges and becomes continuous with the 

 t runks of the vitelline vein s. Connections have also been formed between the 

 descending aortae and the vascular area, but as yet the vitelline arteries have not 



