476 THE PHYSIOLOGY OF REPRODUCTION 



The trophoblast of the beaver in the preplacental stage is at the 

 areas of attachment not more than one layer in thickness. It does 

 not send processes to any depth into the mucosa, but has a flat 

 insertion upon the decidual surface, the maternal capillaries pressing 

 towards the trophoblast rather than otherwise. "Whenever the uterine- 

 epithelium .is displaced by the ob-placental trophoblast it disappears 

 without forming a syncytium. The uterine epithelium probably 

 becomes necrotic in advance of the trophoblastic growth, and the 

 latter does not destroy the epithelium which is normal at the borders 

 of the decidual surface.^ 



Insectivora. — The importance ascribed to the placentation in 

 Inseetivora has already been referred to (see p. 409). The hedgehog, 

 shrew, mole, and Tupaia have been most fully investigated. 



Hedgehog. — In the hedgehog {Erinaceus europmus), the zona 

 pellucida disappears early, before the expansion of the hypoblast, 

 which, as in man, forms a closed vesicle. The chronology of 

 embedding is not yet known. In the earliest stage examined by 

 Hubrecht,^ the blastocyst was 0'22 of a millimetre in diameter. The 

 outer wall was several layers thick all round its circumference, and 

 spaces were already present in it. At a slightly later stage the 

 blastocyst grows rapidly and theepiblast is reduced to a single layer, 

 with numerous villiform processes at intervals, except for a thickened 

 knob which represents the future germinal area. Even now" the 

 name tropliohlast may be given to the single layer of epiblast with its 

 projections, excluding the thickened knob which is formative and 

 gives rise to the embryonic ectoderm and the lining of the amniotic 

 cavity. The mesoblast, as yet one-layered, which extends between 

 the trophoblast and hypoblast, consists of an attenuated somatic part 

 which forms with the trophoblast the diplo-trophoUast,^ and a 

 splanchnic part which forms blood-vessels and blood. 



The early blastocyst comes to rest, as in the mouse, in an anti- 

 mesometrial furrow of the mucosa. It is not yet determined whether 

 any changes occur previously in the uterus ; but at least, soon after 

 the blastocyst has taken up its position, there is a great cell- 

 proliferation in the stroma of the floor and walls of the furrow, not 

 perivascular as in the rabbit, but sub-epithelial. Along with this 

 decidual formation, the lumina of the glands are closed, and their 

 epithelium gradually disappears, perhaps by the influence of the 



1 Willey, "The Blastocyst and Placenta of the Beaver," Quar. Jour. 

 Science, vol. Ix., 1914. 



2 Hubrecht, "The Placentation of Erinacem europceus," Quar. Jour. Mim: 

 Science, voL xxx., 1889. 



2 Hubrecht restricts the term chorion to Tardus (a lemur), monkeys, apes, 

 and man (see p. 490). 



