FCETAL NUTRITION: THE PLACENTA 371 



these cells, may reach the ovum and nourish it (see p. 127). 

 Whatever the source of the food-supply of the ovum is, it not 

 only increases in size until it is ripe for deliverance, but stores 

 in its protoplasm yolk granules, the devtoplasm of Beneden, 

 which increase in number as the ovum approaches maturity. 

 The granules vary in size and number in different species, and 

 also in their position. They may be mingled uniformly through 

 the cytoplasm, or be collected at the njarginal zone (sheep), or 

 at the periphery of the central zone (Man). During the earhest 

 stages of segmentation, when perhaps food is not readily acces- 

 sible, or a specialised form of nutriment is required, the granules 

 are used up- 



II. The Fertilised Ovum and its Coverings 



When the ovum leaves the ovary it carries with it the zona 

 pellucida and cells of the corona radiata. After fertihsation, 

 which most probably occurs in all animals at the outer end of 

 the oviduct or Fallopian tube, the cells disappear and are re- 

 placed in some species by a homogeneous sticky layer of 

 albuminous material. According to Robinson,^ it is derived in 

 part from the disintegrated cells of the corona radiata, but 

 most of it seems to be obtained from the secretion of the 

 tubal and later of the uterine glands.^ It is covered by 

 villous tufts, which led to its designation as prochorion by 

 Hensen. But the tufts are merely casts of the gland-ducts, due 

 to the coagulation of the secretion by the use of reagents. 



The investment formed by the two layers around the ovum 

 is very thick in Marsupials. In Ungulates it forms a thin coat, 

 which disappears at a comparatively early stage in the pig, 

 sheep, and deer. In the last named, according to Bischoff, 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 mesoderm 



' Robinson, " On the Early Stages of Development of Mammalian Ova, 

 and on the Formation of the Placenta," Hunterian Lectures, Jour, of Anat. 

 and Physiol., vol. xxxviii., 1904. 



" Bonnet, " Ueber das PrOohorion der Hundekeimblase," Anat. Anz.. 

 vol. xiii., 1897. 



