1882.] 



Blastodermic Layers in the Hat and Mouse. 



431 



The Decidua. 



So far as the history of this structure is concerned, the following 

 description will answer equally well for any one of the three animals 

 under consideration. 



It is formed by the transformation of the tissue lying between the 

 epithelium of the uterus and the muscular wall. This change is 

 accompanied by the total disappearance of the uterine glands, and 

 consists in the formation of a large mass of round-celled material, 

 occupying the whole circumference of the uterine tube in each 

 loculus. The centre of this decidual mass is occupied by a prolonga- 

 tion of the uterine cavity formed by the extension of the decidual 

 substance round it, while the main cavity of the uterus remains for a 

 time continuous throughout the whole length of the uterine tube. 

 This continuity is interrupted about the ninth day by the obliteration 

 of the cavity caused by the increased decidual growth, with the ex- 

 ception of that part of the cavity in the centre of the decidual mass 

 in which the ovum is situated. 



In the early stage the ovum is equally surrounded by decidua 

 on all sides, but as development proceeds the decidua at the free 

 side of the uterus gradually diminishes and disappears. Before 

 this takes place a separation between the decidua and the muscular 

 wall of the uterus occurs in this region, but not until about the six- 

 teenth day, and the slight connexion which then exists between the 

 decidua and the free side of the uterus can be recognised as a white 

 line passing transversely over the pregnant loculus on removing it from 

 the abdominal cavity. In this manner the uterine cavity becomes 

 again continuous throughout the whole length of the uterine tube. 

 The changes taking place in the decidua at the mesometrial side of 

 the uterus depend upon the formation of the maternal portion of the 

 placenta ; in this region the decidua becomes vacuolated, these vacuoles 

 forming the maternal vessels, lined at first by a single layer of flat 

 cells, and which with the blood-vessels of the foetal portion of the 

 placenta soon come into intimate relation. 



This decidua, present in the three animals under consideration, 

 differs remarkably in its history and extent from that of other 

 Mammals in which the development of the blastodermic layers is 

 regular, and it would appear that there is a close association between 

 the form and extent of this decidua and the peculiar modification 

 of the ordinary type of development which I am now about to 

 describe. 



The Ovum. 



Of the many pregnant loculi of which I have made sections between 

 the sixth and seventh day, the one which affords the key to the expla- 

 nation of this deviation from the ordinary type of development is from 



