THE EARLY DEVELOPMENT OF THE MAMMAL 421 



1. Implantation 



Through the process of menstruation, or procestrous, its 

 equivalent in the lower Mammals, the mucous membrane 

 lining the uterus is kept in an active, wholly living condition, 

 and as the ovum or blastodermic vesicle enters this cavity it 

 almost immediately becomes attached or implanted upon the 

 wall. It may become superficially attached to the wall of 

 the main uterine cavity, so that as it grows it projects freely 

 into the lumen of the uterus; this is known as central implanta- 

 tion and is found in the Ungulates and Carnivors, the lower 

 Primates and some Rodents such as the rabbit. In other 

 forms the vesicle may come to lie in a furrow or groove in the 

 uterine wall, which is then closed off from the main cavity by 

 the fusion of the margins of the furrow, enclosing the vesicle. 

 This is eccentric implantation and is found in such forms as 

 the mouse and some Insectivors. Or the vesicle may burrow 

 into the substance of the mucous membrane lining the uterus, 

 the mucosa then closing together over the point of entrance, 

 as in man and some Rodents, such as the guinea pig and the 

 gopher. This type of implantation is known as interstitial. 



The structure primarily concerned in effecting this early 

 connection between the vesicle and the uterine mucosa is the 

 trophoblast (Hubrecht). We have already described the forma- 

 tion of this superficial layer of ectoderm cells which covers the 

 entire blastodermic vesicle as this passes down the oviduct 

 and enters the uterus (Figs. 154, A; 175). The trophoblast 

 may remain for a short time the only component of the periph- 

 eral wall of the blastodermic vesicle. But extra-embryonic 

 mesoderm usually forms very early around the wall of the 

 vesicle, and the entire extra-embryonic wall may then be 

 known as the chorion; the trophoblast may then be called the 

 chorionic ectoderm. 



It is convenient to distinguish two general types of behavior 

 on the part of the trophoblast or chorionic ectoderm. In 

 most instances of central implantation (e.g., pig, horse) it 

 merely forms an adhesive layer which comes closely into 



