484 A. A. W. HUBRECHT. 



which will enable me to study the placentation process of yet 

 five other genera of Mammalia hitherto not examined. It is 

 preferable to reserve the discussion on points of criticism and 

 on general conclusions, and to describe for the moment only 

 the facts and phenomena as they present themselves in every 

 one of the genera separately. 



B. The Process of Placentation in Outlines. 



The following paragraph, preceding those in which the 

 detailed observations are recorded, is meant to be a recapitula- 

 tion of the general features by which the shrew's placentation 

 is characterised. It precedes the full record of the details 

 instead of closing it, because the former will be better under- 

 stood and more easily grasped if certain general notions have 

 first been discussed. 



The placentation of the shrew is brought about by processes 

 that take place in the maternal tissue, and by processes that 

 affect the blastocyst. At the outset these two sets of processes are 

 quite independent of each other ; later on, when the blastocyst 

 has come to adhere against the maternal tissue, they are closely 

 related; still later the participation of the mother towards 

 the constitution of the ripe placenta is again reduced to the 

 maternal blood by which this organ is permeated. 

 The maternal processes are — 



1°. Unexpected and somewhat peculiarly shaped local dis- 

 tensions of the uterine wall, accompanied by changes in the 

 distribution of glandular tissue, &c., in this wall. 



2°. Considerable local proliferations of maternal uterine 

 epithelium. 



The embryonic processes are — 



1. Local changes in the outer wall of the blastocyst. 



2. Special development of certain portions of the trophoblast 

 which finally constitute a syncytium, in which the allantois- 

 villi and the embryonic blood are in the closest contact with 

 maternal blood, the latter circulating in spaces of embryonic 

 issue without any endothelial lining. 



