DEVELOPMENT OF SALPA. 401 



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 whieli 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 i-elation 

 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 2)arent 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 tlie embryo ; its cavity being a jiart of the 

 original "j)"mitive 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 liomologous in struc- 

 ture, with the vertebrate placenta. 



When the embryo of the solitary Salpa is nearly one 

 millimetre {-^ 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." 



