STUDIES IN MAMMALIAN EMBRYOLOGY. 495 



marked progressive development in the maternal tissues, the 

 phase we now enter upon is one in which embryonic prolifera- 

 tions play the by far more considerable part. Proliferations of 

 the outer layer of trophoblast^ more fully to be described in the 

 next paragraph, are henceforth seconded by vascular develop- 

 ment, first in the omphaloidean region (the vessels of the area 

 vasculosa), later on in the tissues of the allantois^ which make 

 use of the roads that have been opened up by the trophoblast 

 in the future placental regions. 



We may summarise the characteristic features of this new 

 phase as follows : 



1. In the uterine tissue — 



(a) The newly formed epithelial crypts are slowly but 

 gradually invaded by trophoblastic protuberances and ex- 

 crescences^ which exercise both histolytical and vasifactive 

 functions. 



(b) The lateral cushion-shaped proliferations of maternal 

 epithelium against which the trophoblast of the area vasculosa 

 is applied undergo a decided histolytical resorption^ and finally 

 disappear. 



2. In the blastocyst — 



(a) The amnion is formed. 



(b) The area vasculosa on the yolk-sac is completed. 



(c) The allantois originates. 



{d) The trophoblastic annulus and the trophoblastic pro- 

 tuberances noticed sub 1, a, make their appearance. 



It will be seen from the above that the chief feature that is 

 novel to and characteristic of this phase is the intimate fusion 

 over a very extensive surface of embryonic and maternal histo- 

 logical elements, coupled with interesting and as yet only 

 imperfectly understood histolytical and histogenetical pro- 

 cesses. This makes it impossible to describe any further what 

 was set down for this paragraph^ viz. "the changes in the 

 uterine tissues," without making continual references to the 

 growth of the embryonic trophoblast. It is for that reason 



