438 THE PHYSIOLOGY OF REPRODUCTION 



the ectoplacenta. In this way the ovum is completely shut off 

 in a decidual cavity, the " Eikammer," from the uterine lumen. 

 The roof of the chamber forms the primary decidua reflexa, 

 and it is gradually thickened by a decidual deposit. In it new 

 blood-vessels are developed, and they form a specially rich 

 vascular network. 



By this time the blastocyst has become tubular in shape, 



and it shows an inversion of 

 the germinal layers (Fig. 105). 

 In the earher stage a cavity 

 appears in the inner mass of 

 cells. The roof of the cavity 

 becomes thickened to form the 

 " Trager " or ecto - placental 

 cone, which is at first cyhndrical 

 and later conical, with its base 

 resting on the mesometrial pole 

 ~ of the ovum. By its inward 

 growth, it shoves before it the 

 floor of the inner mass consist- 

 ing of epiblast and hypoblast. 

 In this way an invagination is 

 produced in the tube with the 

 epiblast internal to the hypo- 

 blast. Hence the germinal 

 layers are said to be inverted. 



Blood is regularly found 

 in the implantation cavity. It 

 completely surrounds the ovum, 



Fig. 105. — Inversion of the germinal 

 layers in the blastodermic vesicle 

 of the mouse. The trophoblast 

 becomes greatly thickened and 

 Invaginated, pushing the for- 

 mative epiblast before it. The 

 whole blastocyst assumes a tubular 

 shape, and the hypoblast appears 

 to be external to the epiblast. 

 Trophoblast represented by con- 

 tinuous black lines or masses : 

 entoderm by interrupted lines : 

 embryonic ectoderm by epithelial 

 cells. (T. H. Bryce, in Quain'a 

 Anaiotny, Longmans.) 



and reaches irregular spaces in 

 the ectoplacenta which communicate with the surface. At 

 this time, however, there are no foetal vessels near the cone, 

 and the blood in its meshes may be of use only for its own 

 nutrition. On the other hand, the thin trophoblast of the 

 wall of the invaginated yolk-sac is partly vascularised by 

 viteUine vessels, by means of which the nutriment absorbed 

 from the blood effusion may reach the embryo, or be 

 stored in the yolk-sac. In the trophoblast itself the 

 haemoglobin of laked corpuscles and its derivatives are 



