300 COMPARATIVE ANATOMY 



wall in Marsupials, and although our knowledge of the 

 different forms of placentation in this order is far from being 

 complete, we can no longer make a distinction between the 

 Marsupials as non-placental and the Eutheria as placental 

 mammals. 



Our knowledge of the structure of the placenta in the 

 highest mammals or Eutheria is, however, far more complete, 

 and in this group the embryos are retained for a considerable 

 time in the uterus of the mother, and during that time are 

 attached to the uterine wall and nourished by the maternal 

 blood. The nature of this attachment may be briefly 

 described as follows : — 



The ovum is fertilised in the oviduct, and the segmentation 

 is total and equal, resulting in the formation of a solid mass of 

 cells called the blastocyst, which soon becomes differentiated 

 into an outer layer and an inner mass, as shown in fig. 75, A. 

 The outer layer is called the trophoblast, and it does not give 

 rise to any part of the embryo, but is solely concerned with the 

 nutrition of the embryo. The inner mass gives rise to the 

 embryo, the yolk-sac, and the amnion. 



In the next stage (fig. 75, B) a cavity, the blastocystic 

 cavity, is formed between the trophoblast and the inner mass, 

 so that the latter comes to lie at one pole of the now elongated 

 blastocyst. At the same time the cells of the inner mass are 

 differentiated into an embryonic knob above, and a flattened 

 epithelium below. The latter, henceforth known as the 

 epithelium of the yolk-sac, now begins to spread around the 

 blastocystic cavity, forming a lining to its walls. 



In the next stage (fig. 75, C) the upper pole of the tro- 

 phoblast, above the embryonic knob, becomes enormously 

 thickened to form a very important organ generally known 

 by the German name "trager," literally the "carrier." The 

 epithelium of the yolk-sac has spread further round the 

 cavity of the blastocyst, and the embryonic knob has in- 

 creased greatly in size, pushing down the yolk epithelium and 

 forming a large projection into the blastocystic cavity. A 

 considerable space is shortly afterwards formed in the middle 

 of the embryonic knob. 



In the next stage (fig, 75, D) the trager has increased very 

 much in size and has become channelled by numerous lacunae. 

 The yolk epithelium forms a complete lining to the blasto,- 



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