88 



THE DEVELOPMENT OF THE CHICK 



is a layer of the mesoderm (Fig. 41). If the blood-islands be 

 reckoned as mesoderm we must distinguish two layers of the 

 latter, viz., a deep or vascular layer (angioblast) lying next the 

 germ-wall, and an upper layer next the ectoderm, which may 

 be called the coelomic mesoderm, inasmuch as the body-cavity 

 (coelome) develops within it later. 



r" 



pr a. 



5t- 





.i^. 



\ ^^ 



• _*.,i«'»- 



nF- 



prstr- 



'— VylJ>.f^*• 





fit. 



a.p. 



- <9. vasc^ 



-:7 



. • » "^C t J'^ J ~i "-t r» Jr 



■jr^.' 



li^ . . ._ - - -■— — « 



Fig. 45. — Blastoderm and embryo at the stage of four- 

 teen somites. The horns of mesoblast are on the point 

 of meeting in front of the head. 



a. p., Area pellucida. a. vase, Area vasculosa. a. v. i.. 

 Area vitelhna interna. Ht., Heart, n. F., Neural folds. 

 pr'a., Proamnion, pr. str., Primitive streak. S. t., Sinus 

 terminalis. 



There are two sharply contrasted views concerning the origin 

 of the mesoblast in the area opaca. According to the one point 

 of view it is simply a peripheral extension of the primitive streak 

 mesoblast with which as a matter of fact it is continuous (Hert- 

 wig, Rabl, and others). According to the other point of view 



