428 THE PHYSIOLOGY OF REPRODUCTION 



sending protoplasmic processes l^etween the cells (Tig. 113). These 

 processes may even reach past the epithelium to the underlying layer 

 of dilated capillaries (Eoljinson), and absorlj nutritive material or 

 effect gaseous exchanges. 



The trophol)last is single-layered throughout, and, after the first 

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

 whicli in the main follows its lidges and furrows, liut occasionally 

 liridges across a fold. It is vascularised by tlie vessels of the 

 allantois, which completely surrounds the embryo. The gland- 

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

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

 stage of apposition except for the protoplasmic extensions of tlie 

 tropliol)last (Fig. 114). 



Blv 



mes — -. 



Fui. 113. — Section througli the wall of the uterus and the blastocyst of the 

 pig at the twentieth day of piegnancy (Assheton). 



mes, Mesoblast ; Bl.i\ fcetal vessel ; Ti\ trophoblast ; Ep, long columnar 

 epithelium of uterine sui'face. 



The uterine mucosa contains no special cotyledonary areas such 

 as are found in tlie 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 eaiiy stages, and probalily during the whole of pregnancy. As 

 in tlie sheep and one of the lemurs {Gdlajjo iM/isi/inlxoiiis), the glandular 

 orifices are covered Ijy domes of trophoblastic cells, wliich absorb the 

 secretion and transmit it as nutriment for the developing embryo Ijy 

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

 early in pi-egnancy it Ijegins to increase in thickness by a widening 

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

 The constituents of the nutruiient provided for the enrltryos are 

 referred to later (see p. 432). 



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



