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 earlier 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 cylindrical 

 and later conical, with its base 

 resting on the mesometrial pole 



of the ovum. By its inward 

 FIG. 10o. Inversion of the germinal . r, . 



layers in the blastoderraic vesicle growth, it shoves before it the 



of the mouse. The trophoblast floor of the inner mass consist- 



becomes greatly thickened and j of ep ibl a st and hypoblast. 



invaginated, pushing the for- T ,, . . ... 



matfve epiblast before it. The In tms wav an Imagination IS 



whole blastocyst assumes a tubular produced in the tube with the 



shape, and the hypoblast appears epiblast internal to the hypo- 



11 -rr J.T, _ ; ! 



blast. Hence the germinal 

 



to be external to the epiblast. 



Trophoblast represented by con- . 



tinuous black lines or masses : layers are said to be inverted. 



entoderm by interrupted lines : 



embryonic ectoderm by epithelial 



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

 Anatomy, Longmans.) 



Blood is regularly found 



m ^ i m pl ant ation cavity. It 



, , , ,, 



completely surrounds the ovum, 



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 

 vitelline 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 



