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 Salpe 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 actuai 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 layerof theembryo; 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 the 
follicle. 
Ann. & Mag. N. Hist. Ser. 6. Vol. xii. 28 
