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 
(ecelome) develops within it later. 
ies 2 =n 6 A eA) 
ee Nes easier Sees SR. CES Se os Sea Shae peers Vai 2 | 
Pia. 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 vitellina 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 
