444 THE PHYSIOLOGY OF REPRODUCTION 



mucosa. Some of the cells show mitoses, the blood-vessels are 

 full, and a few red blood corpuscles may lie between the cells, 

 and also in the foetal ectoderm. During the penetration of the 

 epithelium by the trophoblast, some of the superficial connective 

 tissue cells enlarge. Their nuclei stain more deeply, and the 

 protoplasm of adjacent cells fuses to form a symplasma. The 

 degenerated tissue in its immediate neighbourhood is absorbed 

 ^^^^ by the ectoderm, and the 



^I^HK blastocyst thus comes to lie 



|1 1 1 | in the substance of the mucosa 



j I jl (Fig. 107). According to von 



Spee, the destruction of uterine 

 tissue is effected entirely by a 

 biochemical process ; there is 

 no evidence of absorption of 

 formed elements by phagocy- 

 tosis. 



Round the periphery of the 

 necrotic zone lies a thick layer 

 of large foetal cells, the two 

 together forming the " Implan- 

 tationshof." Later the sym- 

 plasma degenerates further. 

 The nuclei shrink and lose 

 their chromatin, and the proto- 

 plasm becomes fibrillated and 

 granular. Vacuolations appear 

 in the mass, and coalesce to 

 form a space round the ovum 

 filled with clear fluid. In this way the implantation cavity is 

 excavated till it is limited externally by the large cells. Outside it 

 the decidual cells around the vessels survive, while the rest are 

 transformed to a symplasma and absorbed. Hence the wall is 

 sinuous. The dips are, however, filled up in part by newly 

 formed tissue resembling granulation tissue. It encapsules the 

 necrotic zone, and may be looked on, as in the rabbit, as a 

 defence against the advancing ectoderm (see p. 369). 



By this time the ovum has become tubular, with its long 

 axis perpendicular to the long axis of the uterus. It exhibits, 



FIG. 108. Blastodermic vesicle of 

 the guinea-pig, showing inversion 

 of the germinal layers. The 

 blastocyst is tubular, and the 

 formative cell-mass is invaginated 

 as in the mouse, but the thickened 

 trophoblast is not invaginated to 

 so great an extent as in Fig. 105, 

 and the connection between them 

 is lost. Hence the roof of the 

 amnio-embryonic cavity is inde- 

 pendent of the trophoblast. 

 (T. H. Bryce in Quairfs Anatomy, 

 Longmans.) 



