272 EMBRYOLOGY. 



again converted, by an active process of growth, into a normal mucous 

 membrane, whereby its superficial epithelium probably arises from 

 the preserved remnants of the glandular epithelium. 



SUMMABY. 



1. The human ovum establishes itself ordinarily at the base of 

 the uterus (fundus uteri), between the mouths of the two Fallopian 

 tubes, and becomes overgrown by folds of the mucosa and enclosed 

 in a capsule. 



2. The mucous membrane of the uterus is developed into the 

 maternal envelopes of the ovum, the deciduae, which are distinguished 

 as decidua serotina, reflexa, and vera. 



(a) The decidua serotina is that part of the mucous membrane 

 upon which the ovum immediately lies after its entrance 

 into the uterus and on which the placenta is afterwards 

 developed. 



(6) The decidua reflexa is the part that grows around the ovum. 



(c) The decidua vera arises from the remaining portions of the 

 mucous membrane lining the uterus. 



3. In the formation of the deciduae or deciduous foetal membranes 

 the uterine mucosa undergoes profound alterations of structure, and, 

 accompanied by a rapid growth of the uterine glands and a partial 

 disappearance of its epithelium, becomes differentiated into an inner 

 compact and an outer spongy layer. 



4. Out of the wall of the blastodermic vesicle, so far as it is not 

 employed in the formation of the embryo itself, are developed the 

 foetal envelopes of the offspring, which in the main agree with the 

 foetal envelopes of the remaining Mammals in number and the 

 method of their development, but which present in detail important 

 modifications, which are essentially as follows : 



(a) The amnion is closed from before backward, remains united 

 at the hinder end of the embryo with the serosa (subse- 

 quently the chorion) by means of a short pointed pro- 

 longation, and thus contributes to the formation of the 

 so-called belly-stalk of human embryos. 



(6) The allantois does not grow as a free sac into the extra- 

 embryonic part of the body-cavity, but, in the form 

 of a narrow canal, shoves itself along the under surface 

 of the pointed amniotic prolongation to the chorion, 

 and thus furnishes the chief component of the belly- 

 stalk. 



