420 TUNICATA. 



of cleavage ; we are, however, far from having obtained a cleat- 

 insight into the details of the process. In individual cases, cleav- 

 age seems to be unequal. Salensky figured the embryo of Salpa 

 mucronata in the four-celled stage consisting of four equal blast o- 

 meres, but both he and Todako observed an inequality of the 

 blastomeres in S. pinnata and S. punctata at this stage. [Judging 

 from recent investigations (Nos. XIII. and XXIa.), unequal segmenta- 

 tion is the rule rather than the exception in Salpa.] 



The processes of cleavage are here specially difficult to follow in 

 detail because of the cells which become detached from the wall 

 of the follicle and become applied to the embryonic mass (Fig. 210 A, 

 fz), and even wander in between the blastomeres ; these cells are 

 known as the follicle-cells or kalymmocytes. This immigration of 

 cells, which we may compare to the test-cells of the Ascidiacea (p. 336) 

 and the inner follicle-cells of Pyrosoma (p. 390), is so profuse that 

 the blastomeres at the later stages of cleavage seem actually enveloped 

 in them (Fig. 210 B). appearing to be embedded in a matrix of 

 gonoblasts (as Salensky terms them, Xo. 104). Todako, who was the 

 first to notice this multiplication and immigration of the follicle-cells 

 (Nos. 108 and 109), described them as yolk-cells {cellules lecitiches) 

 and holds that they serve for the nourishment of the embryo which 

 forms from the blastomeres. They are said to undergo granular dis- 

 integration, to be taken in and assimilated by the blastomeres, and, 

 finally, to disappear altogether. Salenskv ( No. 104), on the contrary, 

 sees in these cells the actual constituent elements of the future 

 embryo and therefore calls them gonoblasts.* According to him, 

 the large blastomeres, the protoplasm of which soon becomes divided 

 up in a peculiar way, do not subdivide further and are, in general, 

 incapable of any special further development. They are said finally 



* [The recent and exhaustive investigations made by Brooks (No. I.), 

 Heider (No. XIII.), Korotneff (Nos. XVIII.-XXL/.), and Metcalf (No. 

 XXIV.), prove undoubtedly that Salensky was in error when he ascribed 

 a formative rule to the kalymmocytes, since, however much these observers 

 differ from one another in detail, they were all agreed that the organs of the 

 embryo are eventually wholly formed from the blastomeres, the view of the 

 majority being that the kalymmocytes play an entirely passive role in the 

 development of the embryo, being merely nutritive structures. 



The account given in the following pages is largely based upon Salensky's 

 work on the development of Salpa. Unfortunately, the conclusions arrived 

 at by this observer have, in many cases besides the one mentioned above, 

 not been confirmed by subsequent investigators. This renders our account 

 incomplete and, in some particulars, inaccurate, so that the reader will do 

 well to consult the original monographs of Heider, Korotnkff and Brooks. 

 We have, however, endeavoured, in footnotes, to draw attention to the most 

 serious errors. — Ed.] 



