96 



THE EARLY DEVELOPMENT OE MAMMALS. 



The Origin of the Heart. 



At about the time blood-vessels are developing in the area vasculosa, the 

 head of the embryo is found to have grown so much that it projects forward 

 somewhat. This process is illustrated by the diagrams in figure 43, which are 

 intended to illustrate the method by which the so-called separation of the em- 

 bryo and yolk takes place. It is often said that the embryo is constricted off 

 from the yolk. This, however, is incorrect. The size of the connection between 

 the embryo and the yolk remains absolutely about the same, or even increases in 

 dimensions, but the embryo grows rapidly, so that its head end projects forward 

 and later its caudal end also, hence, though the connection with the yolk may 

 remain unchanged, the growth of the embryo causes that connection to appear 

 relatively smaller. In early stages the head of the embryo grows more rapidly 



Fu;. 43. — Diagrams to Illustrate the Separation of the Embryo from the Yolk. 



bl, Blastopore. //, Head of embryo. AtA, Arclienteron or entodermal cavity, ec, Ectoderm. 



than the caudal end (Fig. 43, C). At the cervical end of the anlage of the head, 

 where the tissues of the embryo bend over to join the yolk, a portion of the ccelom 

 is early developed. It extends across the median line. This coelom is the be- 

 ginning of the pericardial cavity. In connection with it the development of the 

 heart occurs. The formation of this organ is probably initiated by an ingrowth 

 of the future cells of the angioblast, which give rise to the endothelium of the 

 heart. The mesothelium of the dorsal side of the primitive pericardial coelom 

 produces the muscular walls of the heart. The early development and primi 

 tive relations of this organ can be understood by the account given in the prac- 

 tical part of the structure of a chicken embryo with seven segments. 



