670 



MAMMALIA. 



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

 (Fig. 290, i.e.), becomes a sac, as a morula .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. 294. — Development of foetal 

 membranes.— After Hertwig. 



Uppermost figure shows up-growth and 

 down-growth of amnion folds. E., 

 Embryo; a./., amnion fold ; a 1 ., amnion 

 proper ; a 2 ., subzonal membrane ; g. t 

 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 (eil.)\ the 

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

 cavity ; sz.m., subzonal membrane. The 

 fourth figure shows the embryo apart 

 from its membranes; ;«., mouth; a., 

 anus. Note umbilical connection with 

 yolk-sac. 



