125 



of the proximal wall of the yolk-sac) we may turn to his des- 

 cription of the development of the raesoraetrically situated pla- 

 centa proper. 



In the first of the above mentioned periods an enorraous pro- 

 liferation of the ectoplacenta takes place, and in this mass lacunae 

 ('lacunes sauguiraaternelles') are forrned in which extravasated 

 maternal blood circulates. In the second period the capillaries of 

 the allantois, which has meanwhile been forrned, penetrate and 

 ramity in the substance of the ectoplacenta; while the ectoplacental 

 tissues on the maternal side of the placenta grown into the 

 maternal bloodvessels, giving rise to the so-called endo-vascular 

 plasmodium. The portion of the placenta which is penetrated by 

 the allantois increases in size, and the allantoic capillaries become 

 radially arranged with regard to the centre of the uterus. 



In the last period the endovascular plasmodium reaches the 

 muscularis, and then broadens out to form a mass of tissue marking 

 the outer limit of the placenta; in it are included vesicular cells 

 of maternal origin. The cavity of the yolk-sac which, by the dis- 

 appearance of its distal wall and of the adjacent ectoplacental 

 and maternal tissues, now directly communicates with the uterine 

 lumen, is carried both up the side of the placenta, and, by the 

 bending in of the edges of the latter, towards the middle, where 

 portions of it are included in the substance of the allantois. 



Since Professor Duval's work, very little has been added to our 

 knowledge of the placentation of the mouse, although numerous 

 publications have appeared dealing with the placenta in other 

 mammals. 



My own work has grown out of an investigation which was 

 commenced some two years ago in Professor Hubrecht's laboratory 

 in Utrecht. I had intended at first merely to study the structure 

 and formation of the megalokaryocytes of the placenta; but came 

 very speedily to the conclusion that it would be necessary for me 

 to undertake an examination of the whole course of development 

 of the placenta. I was quite aware from the very outset of Pro- 

 fessor Duval's work, but hoped that by the use of a larger number 



