372 THE PHYSIOLOGY OF REPRODUCTION 



(Robinson). In Rodents there are differences. In the rabbit 

 (Fig. 76) the albuminous layer is weU-marked while the fertihsed 

 ovum is still in the Fallopian tube ; on the fourth day, when 

 the uterus is reached, it rapidly thins but remains up to the 

 eighth day (Assheton i). In the rat the covering disappears 

 early — usually about the eight-cell stage. In the mole the 



Fig. 76. — Early blastocyst of the rabbit. (From Hertwig's Entwicklungs- 

 geschichte des Menschen und der Wirbelthiere : by permission of Gustav 

 Fischer.) 



a, albumen layer ; zp, zona pellucida ; t, trophoblast ; sc, segmentation 

 cavity ; ec, mass of embryo cells. 



covering is thick, and, according to Heape,^ the albumen layer 

 is applied in the uterus and not in the Fallopian tube. It per- 

 sists, as in the shrew, till the embryonic ectoderm appears on 

 the surface of the ovum. In the hedgehog and bat it dis- 

 appears before the blastocyst is formed, and in Tupaia javanica 

 it may be already absent in the two-cell stage. Little is 

 known of it in the Primates ; in the earliest ovum investigated, 



1 Assheton, "The Attachment of the Mammalian Embryo to the Walls 

 of the Uterus," Qiiar. Jour. Micr. Sci., vol. xxxvii., 1895. 



^ Heape, " The Development of the Mole (Talpa europcea)," &c., Quar. 

 Jour. Micr. Sci., vol. xxiii., 1883. 



