F(ETAL NUTRITION: THE PLACENTA 397 



formed in the allantoic region, and they fit into crypts which 

 are probably lined with maternal epithelium. Between the 

 foetal and maternal tissues in the crypt is a space filled with 

 secretion. The lymphatic system of the mucosa is enormously 

 developed (Kolster 1 ). 



Sheep. In the sheep and cow the poly-cotyledonary type of 

 placenta is found. The form is determined by the presence 

 from an early period, and independently of pregnancy, of 

 numerous prominences or cotyledonary burrs, which project as 

 thickened knobs of the sub-epithelial tissue into the uterine 

 lumen. During pregnancy they form connections with localised 

 proliferations of the trophoblast. The burrs vary in number 

 from fifty or sixty in the sheep to five or six in the roe-deer. 



The ova of the sheep reach the uterus four or five days after 

 coitus, and the blastodermic vesicles remain free till the seven- 

 teenth day. Then the attachment to the mucosal surface 

 begins, and it is completed by the thirtieth day (Assheton). 

 After the ninth day, when the prochorion ruptures, the tropho- 

 blast comes in contact with the uterine epithelium. Apparently, 

 as the result of this, the absorption of nutriment is easier, and 

 the blastodermic vesicle increases rapidly in size so as to fill the 

 uterine horn, or both horns if only one embryo is present. 



Certain changes occur in the mucosa before attachment. 

 The leucocytes, which in the non-pregnant uterus are situated 

 at the base of the lining epithelium, increase in number and 

 penetrate between the epithelial cells. The glandular sacs, 

 situated at the junction of the branches with the main ducts, 

 expand greatly and actively secrete. It is generally held that 

 the surface epithelium is not destroyed, but Assheton has shown 

 that on the cotyledonary burrs it is distinctly degenerated by 

 the seventeenth day, and he has also brought forward strong 

 evidence that it is not subsequently regenerated, but is re- 

 placed by binucleate cells of the foetal ectoderm. 



In the cotyledonary areas of the trophoblast, villi are de- 

 veloped as buds of epiblast, which afterwards contain cores of 

 mesoblast with branches of the allantoic vessels (Fig. 90). They 



1 Kolster, " Die Embryotrophe placentarer Siiuger, mit besonderer Beriick- 

 sichtigung der Stute," Anat. Hefte, vol. xviii., 1902. 



