W. K. BROOKS ON THE GENUS SALPA. 209 



ultimately gives rise to the reproductive organs of the chain-salpa. 

 My own observations show that, in Salpa pinnata at least, it gives rise to 

 these organs and to nothing else, and that it is set apart very early in 

 embryonic life as the embryonic germ, or, as Huxley has termed it, the 

 generative blastema. In the discussion of the literature at the end of 

 this section I shall refer more at length to the fact that recent writers 

 have believed that other structures of the chain-salpa, besides the repro- 

 ductive organs, are derived from it, but I must dispute this, as far at 

 least as Salpa pinnata is concerned, for my own studies show that in this 

 species it gives rise to the testes, the eggs with their follicles, and to the 

 fertilizing ducts, and to nothing more. 



I have not been able to trace its origin further back than the stage 

 of Fig. 6, Plate XXI, where it is sharply defined ; and an examination of 

 Plate XVIII, and of the more highly magnified sections in Plates XVI 

 and XVII, will show what an unfavorable subject salpa is for attempting 

 to trace relations between the blastomeres of the segmenting egg and the 

 germ cells. 



At the earliest stage in which I have found them, Plate XX, Fig. 6, 

 they form a compact, subspherical, granular mass, which is sharply 

 limited on all sides by a well-defined outline, and is closely packed with 

 transparent vesicular nuclei imbedded in a mass of granular protoplasm 

 with faintly-marked cell outlines. 



Each nucleus contains a central group of chromatin granules, and 

 the nuclei themselves closely resemble the embryonic cells of the sur- 

 rounding tissues of the body in size as well as in other respects, although 

 they are quite different from most of the cells of the eleoblast, which are 

 not blastodermic but accessory in their origin, and I have observed 

 nothing which is inconsistent with the natural hypothesis that the 

 germinal cells arise, as they do in other animals, from certain blasto- 

 meres which are derived from the fertilized egg, and that they are in all 

 probability the products of the division of a single blastomere, although 

 the subject belongs properly in the section on the embryology of salpa. 



According to Todarro, the germinal mass is derived from a single 

 large cell, the "primo germoblasto," which migrates from the eleoblast 

 or " glandula germativa," into the young stolon, where it divides and 

 gives rise to a mass of cells, the " cumulo cellulare primativo," which 

 gives rise not only to the germinal mass, but to all the organs of the bodies 

 of the chain-salpae as well. 



The germinal mass is not formed in the stolon, for, as we have seen, 



