No. 3.] THE CELL-LINEAGE OF NEREIS. 369 



change in the character of the segmentation in one direction or 

 another would cause the mesoblast to take its apparent origin 

 in the one case from the entoblast, in the other from the ecto- 

 blast. Supposing these divergent changes to take place in two 

 originally similar forms — there is every reason to believe that 

 exactly analogous changes have taken place — would the two 

 resulting forms of "gastrulas" be equivalent? Is the two- 

 layered larva of Amphioxus equivalent to that of Lopado- 

 rJiynchus ? I think not.^ 



The development of Nereis has been investigated, especially 

 by Gdtte (No. 9), Salensky (No. 21), myself (No. 30), and v. 

 Wistinghausen (No. 31). Gotte describes, in N. Dumerilii, the 

 cleavage, the general features of the gastrulation, the origin of 

 the mesoblast from a single cell (which, however, has probably 

 nothing to do with the mesoblast), and the general history of 

 the free-swimming trochophore. 



Salensky did not observe the early stages of cleavage, but 

 accurately figured some of the later stages and made an admira- 

 ble study of the early larva by means of actual sections. His 

 results differ from those of Gotte on two points of fundamental 

 importance, of which the first relates to the mesoblast-formation, 

 the second to the axial relations of the larva, (i) Gotte describes 

 the mesoblast as arising from a single cell (produced immediately 

 after the eight-celled stage), which, after dividing into two, passes 

 into the cleavage-cavity and gives rise to the mesoblast-bands, 

 precisely as in the Oligochseta and Hirudinea. According to 

 Salensky, on the other hand, the mesoblast is formed by a pro- 

 liferation of the ventral ectoblast near the lip of the blastopore 

 {loc. cit. p. 568). (2) As to the axial relations, Gotte describes 

 and figures the mesoblast-bands in their earlier stages as lying 

 horizontally {i.e. parallel to the prototroch), so that the antero- 

 posterior axis as thus determined is likewise horizontal. Salen- 



1 In a recent interesting paper (No. 26) Watase has called attention to the impor- 

 tance of the precise examination of the early stages of cleavage by tracing the cell- 

 lineage. I am fully in agreement with the views there set forth, though I cannot 

 fully accept his general conclusions (f/^ p. 455), and it appears tome that the rela- 

 tion between this " cytological method " and the study of the germ-layers is not 

 very clearly explained. The phrase " cytological method " is open to some objection, 

 since in common usage the word cytology is applied to the study rather of the internal 

 phenomena of the cell than of its external relations. The terms cytogeny, cytogenetic 

 seem to me more suitable, though perhaps open to criticism on other grounds. 



