1894.] Recent Literature. 691 
embryonic membranes is formed. The inner end of the embryo 
remains exposed to the blood sinus of the chain salp, and from it the 
placenta is formed. The placenta of Salpa is fundamentally different 
from that of the Mammalia. It is merely a portion of the embryonie 
body through which the blood of the chain salp circulates. Jt appears 
to be exclusively a nutritive organ, not respiratory. The stream of 
water constantly passing through the cloaca of the chain salp and 
bathing the body of the embryo, makes a special respiratory organ un- 
necessary. "The placenta performs its nutritive function in a way very 
different from that of the corresponding mammalian organ. In Salpa 
the placental blood current nourishes the placenta itself and causes the 
cells to multiply. The latter migrate into the body cavity of the em- 
bryo, where they degenerate and are used as food. 
The very remarkable character of the egg development is due to the 
peculiar behavior of the follicle. During the segmentation of the egg, 
the follicle undergoes a considerable increase in size. Its cells prolif- 
erate and the follicle assumes ashape, which may be likened to that of 
a mature Graafian follicle of the vertebrate ovary. That is, there is 
a superficial (or somatic) layer of the follicle, connected over a small 
area with a central mass (visceral layer), the two elsewhere separated 
by acavity. The blastomeres, which are forced apart by the growth of 
the follicular tissue, lie in the visceral layer and the region where vis- 
ceral and somatic layers are connected. The follicle now proceeds to 
develop, as if it were going to form the embryo, while the blastomeres 
remain few in number, scattered about in the midst of the mass of fol- 
licular tissue. It is impossible without figures to explain the way in 
which the follicular tissue is folded and-:hollowed out, to form the 
various parts of what appears to be the embryo. It may be said in a 
word that the follicular tissue gives rise to a body, which is a “ simula- 
erum of the embryo.” In this body, pharynx, cloaca, gill and gill- 
slits, are all developed, but are lined with the follicular cells of which 
the great mass of the body is composed. As the various organs are 
outlined in the follicular tissue, the blastomeres take up certain more 
or less definite positions with reference to each organ. Finally the 
blastomeres begin a rapid growth, and in each organ and throughout 
the body they take the place of the follicle cells, the latter degenerating 
and being ultimately used up as food. Thus in fact the Salpa embryo, 
like that of other animals, is derived from the egg cell and not from 
the follicle, as some investigators have held. 
Professor Brooks suggests an explanation, which is probably the 
true one, of the behavior of the follicle in the Salpa embryo. It is 
well known that in many tunicates the follicle cells migrate in between 
