FCETAL NUTRITION: THE PLACENTA 405 



accessible, 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 fertilisation, which 

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

 or Fallopian tube, the cells disappear and are replaced in some species 

 by a homogeneous sticky layer of albuminous material. According 



FIG. 99. Early blastocyst of the rabbit. (From Hertwig's Entwicklungs- 



f'schichte des Menschen und der Wirbelthierc. By permission of Gustav 

 ischer.) 



a, Albumen layer ; zp, zona pellucida ; t, trophoblast ; *<, segmentation 

 cavity ; <??, mass of embryo cells. 



to Kobinson, 1 it is derived in part from the disintegrated cells of the 

 corona radiata, but most of it seems to be obtained from the secret in 

 of the tubal and later of the uterine glands. 2 It is covered by villous 

 tufts, which led to its designation as prochorion by Henson. 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 



1 Robinson, " On the Early Stages of Development of Mammalian Ova, and 

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

 Phys., vol. xxxviii., 1904. 



2 Bonnet, " Ueber das Prochorion der Hundekeimblase,'' Aunt. A HZ., vol. xiii., 

 1897. 



