52 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



During the later stages of foetal life its own mouth is open, its muscles 

 contract, and there is no reason why it should not breathe for itself 

 exactly like an adult. I therefore regard the placenta as a nutritive 

 organ; pure and simple, and it serves its purpose not by the diffusion of a 

 fluid, but by the transportation of solid food into the body of the embryo. 

 From this point of view it is clear that those investigators who have 

 described it as divided into a foetal chamber and a maternal chamber 

 have been misled by an erroneous notion of its function. 



The detachment of the placenta cells has been observed and noted by 

 both Salensky and Barrois, but it has been regarded as a destructive 

 change and as a sign that the organ has served its purpose and has 

 become superfluous. 



It has been assumed that it reaches its perfect form and serves its 

 purpose, and that it then degenerates and breaks down, and no import- 

 ance has been attached to the process of degeneration, as it has not been 

 regarded as significant. 



No note has been made of the very early stage at which degeneration 

 begins, nor of the fact that it is initiated as soon as the embryo begins 

 to grow, and long before it has reached half or a quarter of the size 

 which it is to have at birth. 



This is hard to explain so long as the disintegration of the placenta 

 is regarded as its destruction, but it becomes quite intelligible as soon as 

 we learn that the detachment of the placenta cells, instead of marking 

 the end of its functional life, is actually a manifestation of its useful 

 activity. 



As the figures in Plate XLV show, the strings of cells multiply at 

 their lower ends by direct division of their nuclei, and as the new cells 

 which are thus formed push up towards the top, they grow very large, 

 while their nuclei become filled with diffused chromatin granules. In 

 Salpa hexagona these cells ultimately reach the top of the placenta, 

 where they gradually become elongated and irregular, and then break 

 through into the body cavity of the embryo as the migratory follicle 

 cells which are shown at 29 in the figures. 



While the details are slightly different in Salpa pinnata, placenta 

 cells migrate bodily into the embryo in the same way, and they are 

 shown in many of the figures, as in Plate XVIII, 29, for example. 



The rapid growth of the embryo seems to be most important to 

 salpa, and while we know almost nothing of its birth rate, the quickness 

 with which the surface of the ocean becomes covered with salpse of all 



