396 THE PHYSIOLOGY OF REPRODUCTION 



goes beyond the stage of apposition except for the proto- 

 plasmic extensions of the trophoblast (Fig. 89). 



The uterine mucosa contains no special cotyledonary 

 areas such as are found in the sheep and cow. The surface 

 epithehum, though it shows the degenerative changes already 

 referred to, is apparently never completely destroyed. The 

 glandular epithehum does not at any time show signs of de- 

 generation. 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 



UM MB 



Fig. 89. — Diagram representing a stage in the formation of the placenta of 

 the pig. (From Robinson, "Hunterian Lectures," Jour. Anai. and 

 Phys., vol. xxxviii., 1904.) 



UM, uterine muscle ; MB, maternal blood-vessel ; UG, uterine glands ; 

 UE, uterine epithelium ; FE, foetal ectoderm ; FM, foetal mesoderm. 



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-epithehal 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. 400). 



Mare. — In the mare the details of placental development are 

 not yet known. In the early stages the blastodermic vesicle 

 is attached to the uterine mucosa by the trophoblast covejjng 

 the lower pole of the ovum, and the attachment is aided by the 

 formation of dehcate, nearly parallel ridges (Ewart ^). Vilh are 



1 Ewart, Critical Period in the Development of the Horse, London, 1897. 



