EMBRYO AND EMBRYONIC MEMBRANES 217 



The splitting of the mesoblast is never complete; but on the 

 contrary the undivided margin begins to thicken after the fourth 

 day, and gradually forms a ring of connective tissue that surrounds 

 the umbilicus of the yolk-sac (Figs. 128 and 129). When this 

 ring closes, about the seventeenth day, it forms a mass of con- 

 nective tissue uniting the yolk-sac and albumen-sac. (See 

 below). 



During the first few days of incubation the albumen loses 

 water rapidly, and becomes more viscid, settling, as this takes 

 place, towards the yolk-sac umbilicus. Thus the amniotic sac 

 containing the embryo lies above; beneath the amniotic sac comes 

 the yolk, and the main mass of the albumen lies towards the 

 caudal end of the embryo (Figs. 128 and 129). 



The allantois expands very rapidly in the extra-embryonic 

 body-cavity, and the latter extends by splitting of the mesoblast 

 into the neighborhood of the yolk-sac umbilicus. When the 

 allantois in its expansion approaches the lower pole of the egg, 

 it begins to wrap itself around the viscid mass of the albumen 

 accumulated there. In so doing, it carries with it a fold of the 

 chorion, as it must do in the nature of the case, and thus the 

 albumen mass begins to be surrounded by folds of the allantois 

 with an intervening layer of the duplicated chorion. These 

 relations will be readily understood by an examination of the 

 accompanying diagrams (Figs. 128 and 129). In this way an 

 albumen-sac, which rapidly becomes closed, is established out- 

 side of the yolk-sac, and the two are united by the undivided 

 portion of the mesoblast around the yolk-sac umbilicus. This 

 connection is never severed, and in consequence the remains of 

 the albumen-sac is drawn with the yolk-sac into the body-cavity 

 towards the end of incubation. 



The sero-amniotic connection, which persists throughout incu- 

 bation, has an important effect on the general disposition of the 

 embryonic membranes. It is formed, as we have seen, in the 

 closure of the amnion, by the thickened ectoderm of the suture; 

 this ectodermal connection is, however, absorbed and replaced 

 on the fifth to the seventh days by a broad mesodermal fu- 

 sion, which maintains a permanent connection between amnion 

 and chorion. One important result of this relation is that the 

 albumen-sac, which is formed by the duplication of the chorion, 

 is prolonged by a tubular diverticulum to the sero-amniotic 



