126 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



merely suggestions which have grown out of comparison of the order of 

 the cleavages involved in segregating the germ-lajers. 



The segregation of the ectoblast as three micromeres is apparently 

 not peculiar to Lepas among Entomostraca. The cleavage of certain 

 parasitic Copepoda has close resemblances to that of Lepas as regards 

 number of cleavages involved in the segregation of the germ-layers. 

 In Lerncca, according to Pedaschenko ('93), the ectoblast and mesoblast 

 are separated from the yolk-macromere. (entoblast) by means of four 

 cleavages. It will appear in the discussion of the germ-layers in the 

 following section of this paper, that in the instance just cited the first 

 three micromeres probably contain all the ectoblast with the " second- 

 ary mesoblast," while the fourth is the primary mesoblast ; in this case, 

 then, the number and order of cleavages involved in germ-laj'er segrega- 

 tion would agree with my observations on Lepas. 



In the figures and accounts of the cleavages of various phyllopods and 

 copepods, in whicli the germ-layers appear to be established as early as 

 the thirty-two-cell stage, there are found many suggestions that further 

 investigations may show a close resemblance to the cell-lineage of Lepas. 

 Some examples of such suggestive papers are those of Grobben ('79, '81) 

 on Moina and Cetochilus, Urbanowicz ('86) and Hacker ('92, '97) on 

 Cyclops, and Pedaschenko ('93) on Lerneea ; but in none of these genera 

 are the facts as yet sufliciently well known to warrant close comparison 

 with Lepas, especially since there is much disagreement between the 

 observations of these investigators. At present this mention of a possi- 

 ble resemblance to the cleavage of Lepas can have only the value of a 

 suggestion, which may possibly stimulate compai'ative study of the 

 cleavage of those Entomostraca in which the early segregation of the 

 germ-layers makes it possible to trace the lineage of the cells to the com- 

 plete separation of the germ-layers. 



The cleavage of Lepas has some general resemblances to that of the 

 nematodes. Particularly is there resemblance in the early segregation 

 of the germ-layers ; but, as to the order of cleavage involved in this 

 process, there are great and at present irreconcilable differences. The 

 first cleavage in Nematoda begins the. separation of the germ-layers. 

 Thus the cell aW- contains ectoblast in tJie nematodes as in the cirri- 

 pede, and cdP contains ectoblast and mes-entoblast. The second cleav- 

 age in the nematodes completes the segregation of the mes-entoblast 

 from ectoblast, whereas this is accom])lished by the third cleavage in 

 Lepas. It is obviously impossible to make any comparison of the 

 details of the early development. 



