248 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



chamber or embryo sac, Plate XVIII, Figs. 4 and 8, with only a minute 

 opening into the atrium at the top. 



The difference between Salpa hexagona and Salpa pinnata will be 

 seen by comparing Plate XVIII, Fig. 4, which shows the brood sac in its 

 most perfect condition, and Plate XLV, Fig. 4, which shows the greatest 

 degree of development which it ever reaches in Salpa hexagona. The 

 embryo sac may be rudimentary as it is in Salpa hexagona and Salpa 

 democratica, or fully developed as it is in Salpa pinnata, but I think 

 there can be no doubt that it is strictly homologous in all the species. 

 Salensky believes, however, that its mode of origin is totally different in 

 different species, and he says, page 152, that its relation to the other parts 

 of the embryo of Salpa africana is very interesting, since it is a constit- 

 uent part of the embryo and does not arise from the wall of the branchial 

 sac, as it does in other species. 



It is rather surprising, however, to find a few pages farther on, page 

 157, the statement that his own observations of this species completely 

 confirm Barrois' account of its origin, for Barrois says that it is formed 

 in Salpa africana (maxima) from the walls of the branchial sac, exactly 

 as Salensky himself describes it in Salpa pinnata. 



To recapitulate: the area around the point where the embryo is 

 attached to the wall of the atrium becomes folded about the embryo in 

 such a way as to form an epithelial capsule which covers the surface of 

 the embryo; a supporting ring which forms the side walls of the pla- 

 centa ; and an embryo sac which, in Salpa pinnata and in many other 

 species, completely covers the embryo. The brood chamber, or the space 

 inside the embryo sac, is part of the atrial chamber of the chain-salpa, 

 and in Salpa pinnata it opens into the chamber by a small duct ; while 

 the space between the two folds of the amnion is part of the body cavity 

 of the chain-salpa. Comparison of cut B or Plate XVIII, Fig. 4, with the 

 horizontal sections in Plates XII, XIII and XIV, will show that a hori- 

 zontal section must pass : 1st, through the outer fold 21 of the embryo 

 sac; 2d, through the space y', between the two folds, part of the body 

 cavity of the chain-salpa ; 3d, through the inner fold of the embryo sac, 

 22; 4th, through the brood chamber 25, which is part of the dorsal 

 chamber of the chain-salpa ; 5th, through the epithelial capsule &', and 

 6th, through the embryo in its follicle. 



