822 



PHYLUM CHORDATA : CLASS MAMMALIA 



lacunas grow out into villi, which aid in this earliest connec- 

 tion between mother and offspring. Long before any 



vascular area or fcetal placenta 

 is developed, the outer ecto- 

 dermal wall has the above 

 nutritive function, and de- 

 serves its name of trophoblast 

 (Fig. 492, Tr.). (3) The en- 

 dodermal or inner mass, which 

 is at first a solid aggregate 

 of cells (Fig. 491, i.c.)^ be- 

 comes 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, fol- 

 lowing the contour of the 

 blastocyst wall, forms the 

 lining of the umbilical 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 embrvo at the future navel. 

 (4) Between the ectoderm and 



Fig. 495. — Development of foetal 

 membranes. — After Hertwig. 



Uppermost figure shows up-gro\vth and 

 down-growth of amnion folds. E., 

 Embryo ; a.f., amnion fold ; a.l, amnion 

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

 gut ; y., umbilical vesicle or yolk-sac. 

 The dotted line represents mesoderm ; 

 the dark, endoderm. 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 (y.s.) ; a.c, amniotic cavity ; 

 sz.m., subzonal membrane. The fourth 

 figure shows the embrj'o apart from its 

 membranes : m., mouth ; a., anus. Note 

 umbilical connection with yolk-sac. 



