S42 MAMMALS. 



blastic subzonal membrane, . are included in Hubrecht's 

 term — diplotrophoblast. (5) From the hind-wall of the gut 

 there grows out a hypoblastic sac, the allantois, insinuating 

 itself and spreading out in the space between the two layers 

 of mesoblast. As an outgrowth of the gut, homologous 

 with the bladder of the frog, the allantois is of course 

 lined by hypoblast or endoderm, but it is sheathed ex- 

 ternally by a layer of mesoblast, which it bears with it 

 as it grows. In all placental mammals the allantois, 

 which becomes richly vascular, unites with the subzonal 

 membrane, and therefore with the external epiblast as 

 well, to form the foetal part of the placenta, from which 

 vascular processes or villi grow out and fit into corres- 

 ponding depressions or crypts on the wall of the uterus. 

 [To the mesoblastic wall of the allantois, plus the subzonal 

 membrane, the term " chorion " is sometimes applied, but as 

 the word has been used in many different senses, its aban- 

 donment, except perhaps in human embryology, is almost 

 imperative.] The complex union of allantois with diplo- 

 trophoblast, Hubrecht calls the allantoidean 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 in- 

 ternally, mesoblastic 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 necessary to consider the maternal tissue. 

 The embryo lay at first in a groove of the uterine wall, 

 moored by the preUminary blastocyst villi, which are as it 

 were pathfinders for those subsequently developed from 

 yolk-sac and allantoic regions. At the point of attachment, 

 the mucous lining of the uterus ceases to be glandular, and 

 becomes much more vascular. As the embryo becomes 

 fixed, the blastocyst almost eating its way in, the outer 

 .epithelium degenerates and disappears; below this the outer 



