428 



THE PHYSIOLOGY OF REPRODUCTION 



sending protoplasmic processes between the cells (Fig. 113). These 

 processes may even reach past the epithelium to the underlying layer 

 of dilated capillaries (Robinson), and absorb nutritive material or 

 effect gaseous exchanges. 



The trophoblast is single-layered throughout, and, after the first 

 three weeks, forms a syncytium. Internally to it lies the mesoblast, 

 which in the main follows its ridges and furrows, but occasionally 

 bridges across a fold. It is vascularised by the vessels of the 

 allantois, which completely surrounds the embryo. The gland- 

 mouths lie along the course of the vessels (Assheton). No formation 

 of villi takes place, and the attachment never goes beyond the 

 stage of apposition except for the protoplasmic extensions of the 

 trophoblast (Fig. 114). 



Blv 



mes 



EP 



FIG. 113. Section through the wall of the uterus and the blastocyst of the 

 pig at the twentieth day of pregnancy (Assheton). 



mes, Mesoblast ; BLv, foetal vessel ; TV, trophoblast ; Ep, long columnar 

 epithelium of uterine surface. 



The uterine mucosa contains no special cotyledonary areas such 

 as are found in the sheep and cow. The surface epithelium, though 

 it shows the degenerative changes already referred to, is apparently 

 never completely destroyed. The glandular epithelium does not at 

 any time show signs of degeneration. It secretes actively during 

 the early stages, and probably during the whole of pregnancy. As 

 in the sheep and one of the lemurs (Galago agisymbanus), the glandular 

 orifices are covered by domes of trophoblastic cells, which absorb the 

 secretion and transmit it as nutriment for the developing embryo by 

 the allantoic vessels. The sub-epithelial tissue is gelatinous, and 

 early in pregnancy it begins to increase in thickness by a widening 

 of the lymphatics and blood-vessels and a new formation of capillaries. 

 The constituents of the nutriment provided for the embryos are 

 referred to later (see p. 432). 



Mare. In the mare the details of placental development are not 



