652 



MAMMALIA. 



senses, its abandonment, except perhaps in human embryo- 

 logy, is almost imperative.] The complex union of allan- 

 tois with diplotrophoblast, 

 Hubrecht calls the allan- 

 toidean trophoblast. (6) But 

 in the hedgehog, rabbit, and 

 some other types, there is a 

 mode of embryonic nutrition 

 between that attained by the 

 epiblastic trophoblast and that 

 effected by the final placenta. 

 The wall of the yolk sac, 

 hypoblastic internally, meso- 

 blastic externally, unites with 

 the subzonal membrane, and 

 becomes the seat of villous 

 processes, which through the 

 external epiblast are connected 

 with the uterine wall. Thus 

 is formed what Hubrecht calls ' 

 an omphaloidean trophoblast. 

 Neither omphaloidean nor 

 allantoic villi ever directly 

 interlock with maternal tissue, 

 but always through the agency 

 of the external epiblastic 

 trophoblast. 



(7) It is now time to turn for 

 a little to the maternal tissue. 



FIG. 227. Development of Foetal 

 Membranes. (After HERTWIG.) 



Uppermost figure shows up-growth and 

 down-growth of amnion folds. E., em- 

 bryo; a.f., amnion fold; i., amnion 

 proper; #2., subzonal membrane;^-., the 

 gut ; y. t 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 (#/.) ; the dwindling yolk sac 

 (y-s.) ; a.c., amniotic cavity; s.z.m., sub- 

 zonal membrane. The fourth figure 

 shows the embryo apart from its mem- 

 branes ; in., mouth; a., anus. Note 

 umbilical connection with yolk sac. 



