No. I.] GERM-LAYERS IN CLEPSFME, 129 



meres" correspond to the teloblasts of Clepsine. The two 

 posterior mesomeres represent the mesoblasts, while the four 

 anterior mesomeres probably represent the neuroblasts and the 

 nephroblasts. Vejdovsky lets the anterior mesomeres divide 

 up into micromeres, and fails to connect them with the nerve- 

 cord and the nephridia. The discovery of a full set of telo- 

 blasts in Lumbricus makes it almost certain that such cells are 

 concerned in the formation of the germ-bands of Rhynchelmis, 

 and the mesomeres appear to be the only cells that could here 

 be so identified. Allowing that the mesomeres admit of this 

 interpretation, it is clear that the posterior macromere in Rhyn- 

 chelmis fulfils the same ends as does the posterior macromere 

 in Clepsine. The mesomeres differ from the teloblasts in num- 

 ber and in mode of origin, but agree with them in position, 

 derivation, and purpose. 



According to Vejdovsky (p. 235), the remnant of the pos- 

 terior macromere, after the production of the three primary 

 mesomeres, takes part with the other three macromeres in 

 the formation of the mesenteron. I have not traced any 

 entodermic elements to this macromere in Clepsine ; but, in 

 view of the fact that the mesoblasts persist for some time after 

 they cease to contribute to the germ-bands, it would not be 

 strange to find that their final products were entoplasts. Should 

 this turn out to be the case, the origin of all the germ-layers 

 would be as nearly the same in both eggs as one could well 

 expect. 



The origin of the mesenteron in Rhynchelmis has been well 

 described and figured by Kowalevsky. A glance at his figures 

 is sufificient to show that the mode of origin is essentially the 

 same as in Clepsine. There are three primary entoblasts (four 

 with the remnant of the posterior macromere), differing from 

 those in Clepsine only in breaking up into a number of second- 

 ary entoblasts. The formation of entoplasts takes place in the 

 same manner as in Clepsine, and the residual yolk is finally in- 

 closed in the mesenteron. The whole process is so similar to 

 what I have described in Clepsine that I have been sur- 

 prised to find Hoffmann and Bergh, who must be familiar with 

 the facts, disposed to reject my conclusions. It is hardly 

 necessary to add that this mode of origin is very common 

 among Arthropods; and that here, as in Rhynchelmis, the 



