458 THE PHYSIOLOGY OF REPRODUCTION 



has shown that two specialised areas, the " Haftflecke," 

 exist before the attachment of the trophoblast. They lie 

 one on each side, about midway between the mesome trial 

 and an ti-mesome trial regions, and are recognised by the ab- 

 sence of glandular ducts. The deeper parts of the glands 

 persist till the end of pregnancy, but none open on the modified 

 areas. 



The uterine epithelium again disappears at the points of 

 contact with the blastocyst over the " Haftflecke." There the 

 trophoblast becomes thickened, and its cells enlarge and pene- 

 trate between the epithelial cells, which fuse to form a sym- 

 plasma. This is quickly absorbed by the trophoblast, which 

 continues to thicken, and now shows two layers, plasmodiblast 

 and cytoblast. The outer layer fuses so closely with the de- 

 cidual tissue as to be indistinguishable from it. The capillaries 

 dilate and new vessels are formed, especially in the layers next 

 the ovum. When their endothelium is destroyed, maternal 

 blood enters the trophoblastic lacunae and soon circulates 

 through them. The inter- vascular connective tissue cells pro- 

 liferate and form the trophospongia. The decidual layers 

 outside it become fibrillar, and soon are extremely attenuated. 

 The trophospongia remains longer, but finally it also thins, 

 and at the end of pregnancy there is only a thin rim of maternal 

 tissue left. 



Over the " Haftflecke " the trophoblast is first vascularised 

 by the vitelline vessels, and a temporary yolk-sac placenta is 

 formed. Later the allantois displaces the yolk-sac, and its 

 vessels vascularise the same part of the trophoblast (Fig. 118). 

 " The permanent placenta replaces the omphalic placenta both 

 physiologically and topographically " (Hubrecht). In this respect 

 Tupaia differs from the hedgehog and the shrew. 



Centetes. A peculiar form of placentation has been de- 

 scribed by Strahl 1 in the tenrec (Centetes ecaudatus). A large 

 effusion of maternal blood destroys the centre of the allantoic 

 placenta, and leaves only a peripheral ring. Round the margin 



1 Strabl, "Beitrage zur vergleichenden Anatomie der Placenta," Abh. 

 Senckenberg Naturf.-Ges., 1905. See also Kolleston, "On the Placental 

 Structures of the Tenrec (Centetes ecaudatus}" &c., Trans. Zool. Soc., London, 

 vol. v., 1863. 



