106 



THE EARLY DEVELOPMENT OF MAMMALS. 



free end, and, further, in that the nuclei of the cells are situated near the top of 

 the layer. When the relations of the two epithelia have not been disturbed, it is 

 readily observed that the concavity of each chorionic ectodermal cell receives the 

 convex end of the uterine epithelial cell, so that the two layers are closely fitted 

 together, cell for cell. 



The Trophoblast. 



The trophoblast is the name applied to the special layer of cells developed 

 on the outer surface of the ectoderm of the mammalian blastodermic vesicle. 



Ut.Ep. 



® ^ 



4? 



i^^rM?^^e:A^?^.^T^, 



Conn. 



Fig. 49. — Pig, 15.0 mm., Series 135, Section 58, to Show the Relations of the Chorion to the 



Uterus. 



C^««, Connective tissue of the uterus. jE'r, Chorionic ectoderm, y]/.?.?, Chorionic mesoderm. W..£/, Uterine 



epithelium. Ve, Chorionic blood-vessel. X 35° diams. 



It has as yet been observed only in unguiculates. The trophoblastic layer may 

 be developed over the entire surface of the ovum, or over only a portion thereof. 

 Its principal known function is to destroy the tissues of the uterus of the mother 

 with which it comes in contact. The destruction of the tissue is supposed to 

 serve two purposes: First, to supply nutrition to the embryo. It is from this 

 supposed function that the layer derives its name of trophoblast. Second, to 

 secure the attachment of the ovum to the wall of the uterus. This preliminary 

 attachment is called the implantation of the ovum. In some cases the tropho- 

 blast is developed very early over the surface of the ovum, appearing almost as 



