356 W. L. SCLATER. 



uterus wall has been in each case separated from the vesicle 

 wall, and on opening the fresh uterus the vesicle always comes 

 away by itself. 



The meaning of the cells marked/ in fig. 17 has puzzled 

 me ; I think it highly probable that these nuclei and their 

 adjacent protoplasm are food material for the embryo lying in 

 the vesicle (not shown in the fig.), since the nuclei resemble 

 nuclei found in the embryo, and the mass of cells, if followed 

 through adjacent sections, are found to form a patch or cap of 

 cells lying on the vesicular thickening; the vesicular thickening 

 itself is seen when followed out to be directly continuous with 

 the vesicle wall, so that the theory that the patch of cells (/) is 

 merely a continuation of the vesicle wall, and the vesicular 

 thickening (pi.) a product of the uterine epithelium, seems to 

 me untenable. 



This vesicle swelling persists till quite the end of the uterine 

 life of the embryo as a thickening at the hinder end of the 

 embryonic vesicle. 



The placenta in the case of mammals is a vascular plexus 

 formed by the uterine epithelium, which is in connection with 

 a vascular plexus formed by part of the embryonic membranes. 



In the case of Peripatus imthurni there is certainly, as 

 far as I have been able to observe, no plexus of blood-vessels at 

 all ; and Kennel, I think, makes no mention of this matter. 



But apart from that, since it may be argued that the word 

 placenta can be applied to any uterine nourishing organ, whether 

 vascular or non- vascular, the swelling of Peripatus is alto- 

 gether an embryonic organ, the uterus takes no part in its 

 formation whatever. 



The vesicular thickening of Peripatus is formed entirely by 

 the proliferation of the cells of the wall of the embryonic 

 vesicle, which vesicle is originally derided from the outer wall 

 of what I have called the pseudogastrula, so that it has nothing 

 whatever to do with the wall of the uterus. 



