DEVELOPMENT OF SALPA. 402 
Brooks has studied the mode of development of the female 
and male Salpa spinosa (Fig. 386°). When a Salpa-chain is 
discharged from the body of the asexual Salpa, each indi- 
vidual of the chain contains a single egg which is fertilized. 
by sperm-cells of individuals belonging to some other chain, 
and after passing through the mulberry stage and entering 
the gastrula stage, the germ is in most intimate relation 
with the body of its parent. The vase-shaped gastrula is. 
lodged in a brood-sac. Its body-cavity, originally formed by 
invagination of the ectoderm, opens directly into the sinus- 
system of its nurse, and the blood now circulates in and out. 
of the primitive digestive cavity as well as around the out- 
side of the embryo. But as the embryo grows and fills the 
brood-sac, so that the outer surface of the gastrula becomes: 
intimately connected with the wall of the brood-sac, the 
blood no longer bathes the outside of the embryo. 
At this time the “placenta” is formed. Brooks believes: 
that it originates directly from the blood, ‘by the aggrega- 
tion and fusion of its corpuscles,” not being derived from any 
of the parts of the parent or embryo. Soon after its appear- 
ance it consists of an inner chamber communicating with the 
sinus of the nurse, and having no communication with any 
of the cavities of the embryo ; its cavity being a part of the 
origina “primitive stomach” of the gastrula. It finally has 
two chambers, an inner and outer one, and Huxley describes* 
the fcetal circulation in the placenta, a deciduous organ 
analogous in function, but by no means homologous in struc- 
ture, with the vertebrate placenta. 
When the embryo of the solitary Salpa is nearly one 
millimetre (s45 inch) long, and while still in the brood-sac of 
the parent, the tube which is to give rise to the chain ap- 
*« The blood-corpuscles of the parent may be readily traced enter 
ing the inner sac on one side of the partition, coursing round it, and 
finally re-entering the parental circulation on the other side of the par- 
tition ; while the foetal blood-corpuscles, of a different size from those: 
of the parent, enter the outer sac, circulate round it at a different rate, 
and leave it to enter into the general circulation of the dorsal sinus. 
More obvious still does the independence of the two circulations be 
come when the circulation of either mother or foetus is reversed.” 
