W. K. BROOKS ON THE GENUS SALPA. 281 



He says little about its function, and it is difficult to state his view 

 of its 'structure and origin briefly, both on account of the manner in 

 which he presents it, in detached sentences, which are nowhere com- 

 bined in a general view, and also because he uses the word "placenta" 

 in no less than three ways. In his account of Salpa africana, for 

 example, and in many other places, this word is used to designate the 

 whole organ, as I have used it. In other places, p. 102, p. 128 and p. 384, 

 it is used to designate the parts derived from the epithelial capsule 

 (Epithelhugel and Ectodermkeim), as distinguished from those derived 

 from the follicle, while in other places, pp. 379 and 384, it is used to 

 designate the central structures derived from the follicle as distin- 

 guished from the side walls derived from the epithelial capsule. 



His account shows that its structure varies greatly in the different 

 species, but his description conveys the impression that the difference is 

 fundamental, although his figures show that this is not the case. Thus, 

 for example, his account of the placenta of Salpa democratica, pp. 379 

 and 384, indicates that the whole placenta is formed from the central 

 structures, and that side walls derived from the epithelial capsule (Epi- 

 thelhugel), and comparable to what, in his. account of Salpa pinnata, 

 p. 102, he calls the " placenta," are absent in Salpa democratica, although 

 his figures, especially Plate XXVII, Fig. 3, represent distinct side walls, 

 which, in this figure, are marked Eph. 



The big migrating placenta cells, 29, of my figures, which have 

 migrated from the roof of the placenta into the body cavity of the 

 embryo, are represented in Salpa pinnata, in his Plate XIV, Figs. 37, pin 

 and 37, pin A, but while they are such a conspicuous feature in its 

 development, I can find no account of them in the text. They are also 

 shown in the embryo of Salpa bicaudata in Plate XXVI, where they are 

 marked plz. In his account of this species he says, p. 367, that they are 

 placenta cells, and on p. 370, that they fuse together at a later stage and 

 form a pad which takes the place of the roof of the placenta. 



As regards the structure of the placenta, I believe that careful study 

 of Salensky's memoir proves the correctness of my view that it is funda- 

 mentally different from that of mammals, that it contains only one 

 chamber, which is a diverticulum from the blood system of the chain- 

 salpa, and that it is purely a nutritive organ. 



His account of it in Salpa pinnata, pp. 102, 111 and 123, agrees in all 

 essentials with my own observations, and he shows that there is here 

 only one chamber ; that the central structures and part of the roof are 



