W. K. BROOKS ON THE GENUS SALPA. 249 



SECTION 5. The Fate of the Foetal Membranes. 



I. The embryo sac. This undergoes little change after it is fully 

 developed, and it does not grow with the growth of the embryo. It is 

 formed in Salpa pinnata at a time when the embryo is very small, and 

 growth causes the embryo to push up through its opening, which is thus 

 greatly distended, into the atrium, as is shown in Plate XLI, Fig. 2, 

 and in Plate XXXV, where the entire body of the salpa is outside the 

 embryo sac, which reaches only to the level of the top of the placenta. 

 In fully grown embryos the embryo sac is so small in comparison that it 

 is scarcely discernible with any magnifying power which can be used. 

 At birth the rupture which sets the embryo free takes place, in Salpa 

 pinnata at least, between the lower edge of the supporting ring and the 

 lower edge of the inner fold, which is thus left behind when the embryo 

 with its placenta drops into the pharynx of the chain-salpa, to escape into 

 the water. 



II. The epithelial capsule. My observations on the fate of the epi- 

 thelial capsule are totally different from Salensky's. He says (389) that 

 in Salpa bicaudata and Salpa democratica it is transitory; while in 

 Salpa africana, pinnata, fusiformis and punctata, it becomes converted 

 into the ectoderm of the embryo. I have studied only one of the species 

 in his list, Salpa pinnata, but I have studied Salpa hexagona, which is a 

 representative of the democratica group, and I find that in these two 

 species it is a temporary deciduous structure, as it is also in Salpa afri- 

 cana according to Barrois (6), p. 478, and in Salpa democratica and Salpa 

 bicaudata, according to Salensky. In these species it has no share in the 

 formation of the embryo. It covers it like an ectodermal epithelium, 

 but as the true ectoderm is gradually developed beneath, it is pushed off 

 or moulted. 



In both these species the ectoderm is blastodermic in its origin, and 

 it is formed, as I shall show later on, from certain ectodermal blasto- 

 meres, Plate XVII, Fig. 5, 9 and 9'; Plate XXII, Figs. 3 and 5, a"; Plate 

 XII, Fig. 10, 9', and cuts A and B, A', which at a very early stage of 

 development migrate out of the follicle and come to lie immediately 

 under the epithelial capsule, &'. 



The history of the epithelial capsule is as follows : At first, Plate XI, 

 1, 2, 3, &', it is nearly uniform in thickness and made up of columnar 

 cells, but as the embryo grows it is stretched so much along its sides that 

 the cells here become very flat, Plate XXII, Figs. 1 and 2, &', and Plates 



