372 THE PHYSIOLOGY OF REPRODUCTION 



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

 (Fig. 76) the albuminous layer is well-marked while the fertilised 

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

 the uterus is I'eached, it rapidly thins but remains up to the 

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

 earlv — usuallv about the eight-cell stage. In the mole the 



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

 [icschirhtc des jllfiischcn und dcr Wirbelthierc ; by permission of Gustav 

 Fischer.) 



o, albumen layer ; :p, zona pelluoida ; /, trophoblast ; sr^ segmentation 

 cavity ; cc, mass of embryo cells. 



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

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

 sists, as in the shrew, tiU 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 Ttipaia javanica 

 it mav be alreadv absent in the two-ceU stage. Little is 

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



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

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



- Heape, -The Development of the Mole (TaJpa europcea)," kc. Quar. 

 Jour. Micr. Sci.. vol. xxiii., 1S83. 



