THE EARLY DEVELOPMENT OF THE CHICK 275 



sisting of endothelial and rudimentary muscular layers, freely 

 suspended in a cavity which is the beginning of the pericardial 

 cavity. Anteriorly it is continuous with a short pair of vessels 

 extending into the head-fold; these are the ventral aorta? men- 

 tioned above. Posteriorly the heart is directly continuous with 

 the vitelline veins. 



The dorsal aorta? have meanwhile extended forward into the 

 head, connecting with their cephalic sections which have 

 developed directly within the mesenchyme of the region. 

 These now unite with the ventral aorta? (Fig. 107, B] around the 

 sides of the anterior end of the fore-gut; these connections are 

 the rudiments of the first, or mandibular aortic arches. Thus 

 the embryonic circulatory system is established and connected 

 with the extra-embryonic vessels. 



IV. THE EMBRYO OF ABOUT THIRTY HOURS (10-12 



PAIRS OF SOMITES) 



We may summarize, now, the events of this early period of 

 development by mentioning the structures of the chick embryo 

 at about its thirtieth hour of incubation, when it includes ten to 

 twelve pairs of mesodermal somites (Figs. 107, 108). 



In the freshly opened egg of this age, the circular blastoderm 

 is seen to have extended approximately one-fourth of the way 

 around the vitellus (i.e., its diameter is roughly 25 mm.). The 

 embryo appears as a definite whitish streak, approximately 4 

 mm. in length, enlarged anteriorly, and posteriorly fading 

 gradually into the surrounding area. Thus by far the greater 

 part of the blastoderm is extra-embryonic. Three definite areas 

 may be distinguished in the extra-embryonic blastoderm (Fig. 

 108). (1) Immediately surrounding the embryo is the clear 

 area pellucida, now greatly restricted and not very distinct, 

 save in the head region which includes the proamnion. The 

 area opaca is now clearly divided into (2) the area vasculosa 

 and (3) the area vitellina. The vascular area, save for a slight 

 interruption anteriorly, has grown entirely around the embryo. 

 It has extended both peripherally, over the yolk, andce ntrally 



