CONNECTION BETWEEN EMBRYO AND MOTHER. 705 



or inner mass, which is at first a solid aggregate of cells 

 (Fig. 344, />.), becomes a sac, as a moiula may become a 

 blastosphere. The upper part 

 of this sac forms the lining of 

 the incipient gut, while the 

 lower portion, following the 

 contour of the blastocyst wall, 

 forms the lining of the um- 

 bilical vesicle (cf. the chick). 

 From this vesicle or yolk-sac 

 the embryo becomes folded 

 off, and the connection between 

 the two is narrowed, just as in 

 the chick, into a canal the 

 vitelline duct, which is part of 

 the "umbilical cord," entering 

 the embryo at the future navel. 

 (4) Between the epiblast and 

 the hypoblast of the embryo, 

 the mesoblast develops, split- 

 ting into an outer, parietal, or 

 somatic, and an inner, visceral, 

 or splanchnic layer. The cavity 

 between these is the incipient 

 body cavity. A double fold of 

 somatic mesoblast, carrying 

 with it a single sheet of epi- 

 blast, rises up round about 



FIG. 348. Development of foetal 

 membranes. After Hertwig. 



Uppermost figure shows up-gro\vth and 

 down-growth of amnion folds. ., 

 Embryo ; a.f., amnion fold ; d\., amnion 

 proper; #-., subzonal membrane; g., 

 the gut ; y., umbilical vesicle or yolk- 

 sac. The dotted line represents meso- 

 derm ; the dark, hypoblast. The second 

 figure shows origin of allantois, and the 

 amnion folds have met. The third figure 

 shows increase of allantois (al.~) ; the 

 dwindling yolk-sac (j f s.); a.c., amniotic 

 cavity; sz.m., subzonal membrane. The 

 fourth figure shows the embryo apart 

 from its membranes; ;;/. , mouth; ft., 

 anus. Note umbilical connection with 

 yolk-sac. 



45 



