THE UMBILICAL CORD. 109 



diagrammatic longitudinal sections in figure 43. If we view the longitudinal 

 sections, we see an increasing protuberance of the head and tail ends of the em- 

 bryo, so that the embryo appears more and more separated from the yolk. The 

 process has long been traditionally described as a folding-in of the germ-layers, 

 but this traditional description is incorrect, for the separation of the embryo is 

 really due to the expansion of the embryo, not to the constriction of this connec- 

 tion with the yolk. The diagrams referred to show at a glance how the original 

 width of the communication is retained, while the intestinal canal or embryonic 

 archenteron extends forward and backward. In figure 4.3, A, the archenteron 

 is open to the yolk throughout its entire extent. In B the head has begun to be 

 free, and with it the archenteric cavity has begun to extend forward and forms a 

 distinct cephalic portion, which is entirely within the embryo and is not open 

 directly to the yolk or, as it would be in mammals, into the entodermal cavity of 

 the blastodermic vesicle. In C the tail also has grown forth from the yolk, and 

 the archenteron with it, so that now we have a caudal embryonic digestive canal. 

 By further development the embryo enlarges more and more, but the opening 

 into the yolk-sac remains nearly the same absolute size. 



The relations of the embryo to the yolk in the anamniota are illustrated by 

 the accompanying figure 50, which represents a transverse section through a 

 young stage of the catfish (Amiurus). The section passes through the head of the 

 embryo and shows both eyes and the slender optic nerves, N .op, almost symmet- 

 rically cut on both sides. The yolk, Yk, is a large mass heavily laden with yolk- 

 granules. Between the tissues of the embryo proper and of the yolk-sac there is 

 a direct continuity. Not only can the ectoderm, Ec, be followed around from 

 the embryo over the yolk-sac, but also a layer of mesoderm. The part of the 

 yolk-sac which carries the yolk-grains is, as above stated, a modification of the 

 entoderm. There is no amnion. 



The Umbilical Cord. 



The umbilical cord may be best defined as the tissues connecting the body 

 proper of the embryo with the amnion. It accordingly includes a portion of the 

 body-stalk and a certain extent of the body-wall or somatopleure. In early 

 stages we can hardly speak of an umbilical cord, because the amnion arises 

 close to the embryo (Fig. 69). As development progresses the body-stalk 

 lengthens out (Fig. 40), and the amnion arising from it recedes further and 

 further from the embryo, this recession being assisted by a growth of the somat- 

 opleure which leads to the formation of the umbilical cord, Urn, proper. By 

 this means a tubular structure is produced, the cavity of the tube being a pro- 

 longation of the coelom of the embryo. During the first development of the 



