FGETAL NUTRITION : THE PLACENTA . 407 



not yet pressed on by the investment. Later the vesicle increases in 

 size, and the outer cells are pressed and flattened. At the same time 

 the albumen layer is thinned; and is soon hardly perceptible. Finally 

 it ruptures, and immediately afterwards the blastodermic vesicje is 

 flaccid, apparently from mjury to its wall. 



Besides its nutritive and protective function, the investing layer 

 may prevent the contact of the external cells of the blastodermic 

 vesicle with the cells of the uterus. Only when it has disappeared 

 is fusion of the maternal and fcetal elements possible. Eobinson has 

 followed out this idea in different Mammals. He suggests that in 

 those animals (Carnivores, rabbit) in which the embryonic ectoderm 

 reaches the surface, 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 lies 

 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 little longer in the sheep ; in the guinea-pig it is said 

 to reach the uterus on the seventh day after copulation and while in 

 the morula stage.^ For a further period it remains unattached 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. Peters^ has described a human ovum of five or six days 

 when the decidua was still undergoing differentiation. The ovum 

 was sunk in it but was not as yet completely covered. Leopold^ in 

 a uterus removed surgically for cervical cancer found a seven days' 

 ovum entirely enclosed by the decidua. 



At first each blastomere is nourished separately ; but when the 

 blastocyst is formecl, 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 tropJwUasi,^ 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 



' Von Spee, " Die Implantation des Meerschweincheneies in die Uterus- 

 wand," Zeitsch. f. Morph. vnd Anthropol., vol. iii., 1901. 



^ Peters, Uber die Einbettung des MenschlicJien Eies, Leipzig und Wien, 1890. 

 ' Leopold, Uterus und Kind, Leipzig, 1897. 

 ■• See below, p. 412. 



