DEVELOPMENT OF SALPA. 211 



Brooks has studied the mode of development of the female 

 and male Salpa spinosa (Fig. 140). 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- 

 Gystem 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 

 original "primitive stomach" of the gastrula. It finally has 

 two chambers, an inner and outer one, and Huxley describes* 

 the foetal 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 about four 

 millimetres (-fa 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/' 



