FGETAL NUTRITION: THE PLACENTA 



473 



the non-pregnant uterus two layers are present, a sub-epithelial 

 layer of embryonic connective tissue cells interrupted only by 

 capillaries and glands, and a deeper, more reticulate layer. Before 

 the ninth day of pregnancy, no very marked changes occur in the 

 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 foatal ectoderm. During the penetration of the epithelium by 

 the trophoblast, some of the superficial con- 

 nective 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 blastocyst thus comes to lie in the 

 substance of the mucosa (Fig. 134). Accord- 

 ing 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 phagocytosis. 



Round the periphery of the necrotic 

 zone lies a thick layer of large foetal cells, 

 the two together forming the " Implanta- 

 tionshof." Later the symplasma degener- 

 ates further. The nuclei shrink and lose 

 their chromatin, and the protoplasm be- 

 comes fibrillated and granular. Vacuola- 

 tions 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 exter- 

 nally by the large cells. Outside it the 

 decidual cells around the vessels survive, 

 while the rest are transformed to a sym- 

 plasma 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 lie looked on, as in the rabbit, 

 as a defence against the advancing ectoderm (see p. 403). 



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

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

 mouse, an inversion of the germinal layers, but in the guinea-pig the 

 amnio- embryonic vesicle is closed and separates the thickened tropho- 

 blast from the embryonic ectoderm (Fig. 135). With the growth of 

 the blastodermic vesicle, the roof of the implantation cavity projects 



FIG. 135. Blastodermic 

 vesicle of the guinea- 

 pig, showing inver- 

 sion of the germinal 

 layers. The blasto- 

 cyst is tubular, and 

 the formative cell- 

 mass is invaginated 

 as in the mouse, but 

 the thickened tro- 

 phoblast is not in- 

 vaginated to so 

 great an extent as 

 in Fig. 132, and the 

 connection between 

 them is lost. Hence 

 the roof of the 

 amnio-embryonic 

 cavity is independ- 

 ent of the tropho- 

 blast. (T. H. Bryce 

 in (Juain's Anatomy, 

 Longmans.) 



