THE PLAOENTATION OF PERAMBLKS. 389 



the lengthened allantoic stalk, passes to the exterior, not by 

 way of the lateral vaginal canals, but by breaking through 

 along a median track leading backwards from a posterior 

 common portion of the two uteri. 



Description op Stages. 

 Structure of the Non-pregnant Uterus. 



The uterine wall is shown in transverse section in fig. 1. 

 Externally is the fairly thick serosa {s.) continued on from the 

 ligamentum latum. Internal to the serosa is the muscularis, 

 composed of circularly running non-striate fibres (c. m.). The 

 mucosa (m.) follows immediately on the muscularis, and is on 

 the whole sharply marked off from the latter; occasionally, 

 however, the terminal ends of the uterine glands may pene- 

 trate into the muscularis. 



The mucosa varies considerably in thickness in different 

 uteri, averaging about '75 mm. Its free surface is thrown 

 into irregular longitudinal folds. The matrix of the mucosa 

 consists of fairly compact retiform connective tissue (c. t.), in 

 which are embedded the uterine glands and blood-vessels. 



The uterine glands {gl.) are very numerous, straight or 

 greatly convoluted tubules, averaging '045 mm. in diameter. 

 They are lined by a low columnar epithelium, outside of which 

 is a thin tunica propria derived from the surrounding con- 

 nective tissue. They open freely into the uterine lumen. 



The blood-vessels enter the mucosa from the circular mus- 

 cularis. The majority of the superficial vessels of the mucosa 

 are of the nature of capillaries, with only an adventitious layer 

 of connective tissue surrounding the endothelium ; in the 

 deeper portions of the mucosa, however, vessels with distinct 

 muscular walls also occur. 



The lining epithelium of the uterus {ep.) consists of a layer 

 of low columnar cells with a thickness of about '015 mm., and 

 continuous with the lining epithelium of the glands at the 

 gland openings. 



