468 ZOOLOGY FOR MEDICAL STUDENTS chap. 



an outer of more columnar cells in immediate contact with the trophoblast, 

 and an inner of flatter cells, this latter extending beyond the margin of 

 the columnar cells so as to come also into direct relation to the trophoblast. 

 The blastocyst at this stage when observed as a whole is seen to form 

 a clear-looking vesicle with a dim patch in the neighbourhood of one 

 pole, this dim patch being the portion of the vesicle wall which has the 

 " inner-mass " cells apposed to it. As will have been gathered all of 

 this dim area except its marginal portion consists of three layers of cells. 

 Of these the innermost is endodermal and the two outer ectodermal. Of 

 the latter the outermost is part of the trophoblast while the middle 

 layer will become the ordinary ectoderm covering the body of the embryo 

 and the amniotic folds. Towards the margin of the dim patch only two 

 layers of cells are present — trophoblast and endoderm. The portion of 

 trophoblast in the central three-layered portion is sometimes given a 

 special name — Rauber's layer : it disappears later and plays no part in 

 further development. 



The body of the embryo develops in the dim patch of blastocyst wall 

 very much in the same way as it does in the blastoderm of the bird — 

 primitive streak, medullary folds and so on making their appearance and 

 going through a similar set of changes. In the case of the amnion a 

 difference in detail is seen in the fact that it is here the tailward portion 

 of the amniotic fold that develops most rapidly so that the amnion 

 spreads over the body of the embryo from the tail end forwards. 



During the early stages in its development the egg travels slowly 

 down the genital duct but this progress ceases as it becomes distended, 

 and towards the close of the seventh day the blastocyst becomes anchored 

 to the uterine wall by fine tags or villi which sprout out from the surface 

 of the trophoblast and burrow into the lining epithelium of the uterus. 

 These villi are scattered irregularly over the surface of the blastocyst 

 but there is a special development of them over a somewhat U-shaped 

 area embracing the hinder end of the embryonic body. This special 

 aggregation of tags or villi with the thickened cushion of trophoblast 

 from which they spring is termed the ectoplacenta and it forms the 

 foundation for the development of the true placenta. 



The general arrangement of the parts in the blastocyst about ten 

 days after fertilization is very much the same as in the Fowl's egg 

 during the early days of incubation. We see the same layers of cells, 

 the same parts of the neural rudiment, the amnion, the allantois. There 

 is a large cavity with the endoderm spireading round it exactly like the 

 yolk-sac of the Bird : in fact it clearly is the yolk-sac : but here it 

 contains only watery fluid and no trace of yolk. Even had we no other 



