No. 3.] THE CELL-LINEAGE OF NEREIS. 371 



tial point. The origin and fate of the two " proteloblasts " 

 (called " somatoblasts " by v. Wistinghausen) are the same, and 

 so are the orientation and general history of the germ-bands, 

 though there are certain interesting differences of detail. His 

 orientation of the first three cleavage-planes also agrees exactly 

 with mine, though his terminology is different, owing to a dif- 

 ferent designation of the trochophore-axes. A more extended 

 review of his results will be given further on. 



A comparison of these investigations leaves no room to doubt 

 that Gotte's orientation of the mesoblast-bands and his account 

 of the first origin of the mesoblast are both erroneous. It is 

 barely possible that the free-swimming trochophores of N. 

 Dnmerilii differ from the suppressed trochophores in regard 

 to the early development, but this is extremely improbable in 

 view of the fact that the suppressed trochophores agree pre- 

 cisely with the free-swimming trochophores of three other spe- 

 cies (TV. liinbata, N. megalops, and N. cultriferd). I venture to 

 assert that a comparison of Gotte's figures with those given in 

 the present paper will leave no room to doubt that the cells 

 described by him as "primary raesoblasts " are the posterior 

 proteloblasts (i.e. the first progeny of the first somatoblast, p. 

 407), viewed obliquely in optical section, and that the real 

 primary mesoblasts are figured by him as ordinary ectoblast- 

 cells at the posterior lip of the blastopore. His figures of the 

 horizontal mesoblast-bands I will not undertake to explain. 



In the present paper I shall consider mainly the external 

 aspects of the cleavage and the general relations of the larval 

 and adult bodies. I have devoted my attention mainly to sur- 

 face views, partly because this part of the subject has hitherto 

 been least known and has proved most fruitful of new results, 

 partly because this part of the work demanded so great an ex- 

 penditure of time and labor that I have been forced to defer to 

 a later paper an account of the internal phenomena of cleavage 

 and a detailed study of the differentiation of the tissues and 

 organs. 



Egg-laying. — Methods. 



The eggs of N. limbata and A^ megalops are discharged at 

 night while the animals are actively swimming at the surface of 

 the water. At Wood's Holl, Mass., the season extends at least 



