298 THE CHICK. 



from niesoblast, it should be remembered that the whole of the 

 mesoblast in the chick, with the exception of the primitive streak 

 mesoblast, is of hypoblastic origin ; and the facts with regard to 

 the formation of the blood-vessels might therefore be expressed 

 by saying that the blood-vessels separate from the hypoblast at a 

 stage later than that at which the other mesoblastic structures 

 are formed from it. It is better, however, to accept the facts as 

 they stand ; namely, that in the chick many of the blood-vessels 

 are derived directly and independently from the hypoblast ; and 

 to bear in mind that the middle germinal layer, or mesoblast, can- 

 not be regarded as in any sense equivalent to either of the two 

 primary germinal layers, epiblast and hypoblast ; the term 

 ' mesoblast ' being used to include a number of very diverse 

 structures, most if not all of which owe their ultimate origin 

 to either hypoblast or epiblast. 



The hypoblastic vascular network, formed in the way de- 

 scribed above, gives rise directly to the capillaries and to the 

 endothelial lining of the larger blood-vessels. The connective 

 tissue and muscular walls of these latter are derived indepen- 

 dently from the mesoblast, which grows round and envelopes 

 them. 



The blood-vessels within the embryo are at first, like those of 

 the area pellucida and area vasciilosa, reticular in their arrange- 

 ment. The definite arteries and veins are formed by straightening 

 and enlargement of certain of the strands of the network, with 

 disappearance of other portions ; the dorsal aortas, for instance, 

 arising early in the second day, in embryos with eight pairs of 

 mesoblastic somites, as a pair of longitudinal trunks, lying along 

 the outer and ventral borders of the somites, between the meso- 

 blast and hypoblast, and communicating freely along the hinder 

 part of their course with the reticular network of the area 

 pellucida. 



2. The Heart. 



The heart is formed on the under surface of the fore-gut, at 

 the commencement of the second day. It consists at first of two 

 longitudinal vessels, which, though closely applied in the median 

 plane, ai'e for a time quite distinct, but which soon fuse to form 

 a single median tube. 



The walls of the heart consist, as in the frog, of an outer 



