4o6 THE PHYSIOLOGY OF REPRODUCTION 



disappears at a comparatively early stage in the pig, sheep, and deer. 

 In the last named, according to Bisehoff, there is no albumen layer. 

 In Carnivores there is invariably a firm coat of zona pellucida or 

 albumen layer, or both, which persists, in the dog and ferret at least, 

 till the appearance of the primitive streak and the commencement of 

 the formation of the niesoderm (Robinson). In Rodents there are 

 differences. In the rabbit (Fig. 99) the albuminous layer is well 

 marked while the fertilised 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 daj (Assheton^). In the rat the covering 

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

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

 is applied in the uterus and not in the Fallopian tube. It persists, as 

 in the shrew, till the embryonic ectoderm appears on the surface 

 of the ovum. In the hedgehog and bat it disappears before the 

 blastocyst is formed, and in Twpaia javaniea it may be already absent 

 in the two-cell stage. Little is known of it in the Primates ; in the 

 earliest ovum investigated, the four-cell stage of Macacus netnestrinus, 

 it had already disappeared. 



With regard to its functions, there is little doubt that the 

 degenerating cells of the corona radiata, and later the albumen layer, 

 serve as food for the growing mass of the ovum in the Fallopian tube 

 and uterus. In the investment in the mouse, Jenkinson* found 

 nutritive substances — fat, and probably also protein matter. In 

 addition. Bonnet has adduced strong evidence to show that it is 

 absorbed by the ectoderm of the blastodermic vesicle. In the rabbit 

 the albumen layer forms a tough, strong membrane enclosing at the 

 end of the third day the solid morula. Within the mass of cells a 

 cavity develops and rapidly increases by diffusion inwards of fluid. 

 " It is hardly conceivable that the delicate cells could cause expansion 

 of the tough albuminous wall. Rather the osmotic current is more 

 inwards than outwards, either simple or more probably assisted by 

 the vital activity of the cells" (Assheton). Heape had previously 

 pointed out that the increasing fluid must be secreted into the interior 

 of the blastocyst under considerable pressure, as the vesicle remains 

 spherical and extends the uterine walls before it. Once inside, the 

 fluid exerts a greater or less hydrostatic pressure, which is counteracted 

 by the albumen layer, and the rupture 6f the vesicle is prevented. 

 At the beginning of the cavity-formation in the morula, the cells are 



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

 Uterus," QvMr. Jour. Mia: Science, vol. xxxvii., 1895. 



^ Heape, "The Development of the Mole {Talpa earopcea), etc.," Qiuir. Jour. 

 Micr. Science, vol. xxiii., 1883. 



^ Jenkinson, " Observations on the Physiology and Histology of the Placenta 

 of the Mouse," Tijd. Nederl. Dierk., Ver. ii., Dl. 7. 



