Nutrition of the Salpa Embryo. 373 



down, and no importance 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. 



These 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. 



While the details are slightly different in Salpa pinnata, 

 placenta-cells migrate bodily into the embryo in the same 

 way. 



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 Saljuv of all ages in a long calm shows that the 

 animals are most prolific, and the complicated structure of 

 the organs for nourishing the embryo shows that every pro- 

 vision is made for rapid growth. 



The placenta is not the only nutritive organ, for the follicle 

 also makes most important contributions to the supply of 

 material which is available for the construction and rapid 

 completion of the body of the embryo ; and while I have 

 spoken of the segmentation and the formation of the blasto- 

 dermic germ-layers as retarded, the retardation is probably 

 not actual but only relative, and the process of development 

 is, on the whole, accelerated by the presence of the follicle 

 and by its share in the growth of the embryo. 



The ultimate fate of all the follicle-cells is the same, and 

 they may be found in the sections, detaching themselves and 

 degenerating, first, in the somatic layer of the embryo; secondly, 

 in the somatic follicular lining of the perithoracic structures ; 

 thirdly, in the cavity of the pharynx ; fourthly, in the visceral 

 mass outside the digestive cavity; and, last, in that part of 

 the placenta which is derived from the somatic layer of tiie 

 follicle. 



Ann. & Mag. N. Hist. Ser. 6. Vol. xii. 28 



