444 THE BABBIT. 



in the maternal capillaries of the placenta is specially charac- 

 terised by the relatively enormous number of leucocytes which 

 it contains, at all stages from about the eleventh or twelfth day 

 onwards. 



Curious modifications occur in the lining epithelium of the 

 maternal capillaries of the submucous layer during the forma- 

 tion of the placenta. The ordinary endothelial walls, which 

 these vessels at first have, become replaced by a layer of 

 irregular, thickened, and often columnar cells. According to 

 Duval, this layer is formed by extension outwards of cells from 

 the ectoplacenta along the interior of the maternal vessels ; 

 while Minot regards it as formed by degenerative changes in 

 the proper endothelial wall of the capillaries. 



In the obplacental region, and to a less extent in the peri- 

 placental region, certain peculiar cells, characterised chiefly by 

 their enormous size, and hence spoken of as colossal or giant 

 cells, appear in the submucous layer at an early stage (Fig. 169, 

 UL). These are stated to be derived from the uterine epithelium 

 of these regions ; they form marked features from the ninth 

 day onwards, but their function is entirely unknown. Cells of 

 exceptional size are commonly associated with absorptive, rather 

 than with formative changes, but the actual absorption occur- 

 ring in this region of the uterus is comparatively slight in 

 amount. 



Parturition. The outer layer of the submucous connective 

 tissue, next to the circular muscle layer of the uterus, is charac- 

 terised by the small size of its blood-vessels (Fig. 170, ro) ; and 

 it is along this line that separation takes place at the time of 

 birth, the entire placenta, both foetal and maternal, coming away 

 with the young animal. 



The actual separation is effected by strong contractions of 

 the muscles of the uterus. The haemorrhage at parturition is 

 but slight, partly because the blood-vessels along the plane of 

 separation are small, and partly because of the rapidity with 

 which complete contraction of the uterus is effected . 



Long before the birth of the young animal, the mucous 

 membrane of the obplacental region of the uterus has been 

 completely re-established ; this mucous membrane is attached 

 to the muscular walls of the uterus by very loose connective 



