CONNECTION BETWEEN EMBRYO AND MOTHER. 729 



the future inner layer, endoderm or hypoblast (Fig. 398, L). 



(2) The epiblast divides into an embryonic disc, which will 

 form the epidermis, nervous 



system, etc., of the embryo, 

 and an external layer, the wall 

 of the embryonic sac or blasto- 

 cyst, with which the disc retains 

 a slight connection until the 

 protective amnion is formed. 

 In the outer epiblastic wall 

 lacunae develop, which are 

 bathed by the maternal blood, 

 and the pillars of tissue 

 between the lacunae grow out 

 into villi, which aid in this 

 earliest connection between 

 mother and offspring. Long 

 before any vascular area or 

 foetal placenta is developed, 

 the outer epiblastic wall has 

 the above nutritive function, 

 and deserves its name of 

 trophoblast (Fig. 396, Tr.). 



(3) The hypoblast or inner 

 mass, which is at first a solid 

 aggregate of cells (Fig. 395, 

 i.e.), becomes a sac, as a 

 morula may become a blasto- 

 sphere. The upper part 



FIG. 399. Development of foetal 

 membranes. After Hertwig. 



Uppermost figure shows up-growth and 

 down-growth of amnion folds. E., 

 Kmbryo ; fi.f.^ amnion fold ; a. 1 , amnion 

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

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

 The dotted line represents mesoderm ; 



the dark, hypoblast. The second figure 

 shows origin of allantois, and the amnion 

 folds have met. The third figure shows 

 increase of allantois (a/.); the dwindling 

 yolk-sac (ys.) ; a.c., amniotic cavity ; 

 sz.m., subzonal membrane. The fourth 

 figure shows the embryo apart from its 

 membranes : /#., mouth ; ., anus. Note 

 umbilical connection with yolk-sac. 



