374 THE PHYSIOLOGY OF REPRODUCTION 



the albumen layer prevents contact with the uterine wall till 

 differentiation of the ectodermal cells has taken place to such 

 an extent that they are no longer disposed to fuse with the 

 uterine tissues. In those in which the embryonic ectoderm 

 never reaches the surface (mouse, guinea-pig, hedgehog, bat, 

 probably Primates), the investment disappears before the 

 blastula is attained. 



With the disappearance of the zona, the developing ovum 

 hes naked in the Fallopian tube or the uterus. It takes some 

 time to complete the journey along the tube — about eighty 

 hours in the rabbit, and a httle longer in the sheep. For a 

 further period it remains imattached in the uterine cavity, 

 and then, by processes which vary in different orders, it obtains 

 attachment — loose in Marsupials and firmer in the other orders. 



At first each blastomere is nourished separately ; but when 

 the blastocyst is formed, the greater part of its outer layer is 

 set aside to look after the nutrition of the whole, and takes no 

 share in the formation of the embryo or amnion. To that part 

 Hubrecht gave the name of trophoblast, and the term has been 

 generally accepted. Already, before the embryo is elaborated, 

 provision is in this way made for its maintenance. 



III. The Uterine Mucosa 



While the ovum is still in the oviduct, no obvious changes 

 occur in the uterus itself. In the sheep, Assheton ^ detected no 

 difference except an increase in the number of the leucocytes. 

 There was no sign of activity in the uterine glands or blood- 

 vessels. When the ovum reaches the uterus changes begin — 

 dilatation of blood-vessels and lymphatics, widening and in- 

 creased tortuosity of glands, disappearance of ciha from the 

 surface epithehum. The whole mucosa is soft and oedematous, 

 and there may even be a transudation of lymph into the uterine 

 cavity, which is mingled with the glandular secretion to form 

 a supply of nutriment for the ovum before attachment. Great 



1 Assheton, "The Morphology of the Ungulate Placenta," &c., Phil. 

 Trans. Roy. Soc, London, Ser. B., vol. cxcviii., 1906. 



