468 THE PHYSIOLOGY OF REPRODUCTION 



By tliis time the blastocyst has become tubular in shape, and it 

 shows an inversion of the germinal layers (Fig. 132). 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 ectoplacental 

 cone, which is at first cylindrical 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 consisting of epiblast and hypoblast. 

 In this way an invagination is produced in 

 the tube with the epiblast internal to the 

 hypoblast. Hence the germinal layers are 

 said to be inverted. 



Blood is regularly found in the im- 

 plantation 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 fcetal 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 in- 

 vaginated 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 

 hfemoglobin of laked corpuscles and its 

 derivatives are present (Jenkinson 1 ), and 

 the contents of the umbilical vesicle are 

 "not yolk, but another nutritive substance 

 which the ovum, in the -absence of yolk, 

 takes from the maternal tissues, viz. haemo- 

 globin " (Sobotta 2 ). 



The clecidual cavity is at first small and 

 ovoid, and has a thick wall. As it grows, 

 the lumen of the 'uterus is obliterated, 

 and at its point of contact with the mesometrial wall the 

 epithelium of the latter disappears. Thereafter the two layers 

 fuse, and at the point of fusion the placenta is developed. The 

 lumen of the uterus is later re-established, as in the guinea-pig (see 

 Fig. 137), at the floor of the decidual cavity. Hence the primary 



1 Jenkinson, " Observations on the Histology and Physiology of the Placenta 

 of the Mouse," Tijd. Nederl. JJierk., Ver. ii., Dl. vii. 



2 Sobotta, "Die Entwicklung der Maus," Arch.f. mikr. Anat., vol. Ixi., 1903. 



FIG. 132. Inversion of 

 the germinal layers 

 in the blastodermic 

 vesicle of the mouse. 

 The trophoblast be- 

 comes greatly 

 thickened and in- 

 vaginated, pushing 

 the formative epi- 

 blast before it. The 

 whole blastocyst 

 assumes a tubular 

 shape, and the 

 hypoblast appears 

 to be external to the 

 epiblast. Tropho- 

 blast represented 

 by continuous black 

 lines or masses : 

 entoderm by inter- 

 rupted lines : em- 

 bryonic ectoderm 

 by epithelial cells. 

 (T. H. Bryce, in 

 Quoin's Anatimii/, 

 Longmans.) 



