6 1 2 WILSON. [Vol. VIII. 



Let us attempt finally to apply this general conception to 

 the highly differentiated types of cleavage, such as we find, 

 for example, in the annelids. These forms have been prac- 

 tically ignored by Hertwig and Driesch. Neither of these 

 authors has given sufficient attention to the fact that the 

 period of differentiation varies in different cases, and both 

 have been thus led, as I believe, to a one-sided and premature 

 judgment of the mosaic theory. 



In A^npJiioxns the first visible differentiation takes place at 

 the third cleavage, in Polygordius and EcJiinus at the fourth, 

 in Synapta at a much later period. In the tunicate Clavelina 

 it occurs at the second cleavage, and in many annelids at the 

 first. In Nereis, for example, which is the best-known form, 

 the first division of the Q.g% is unequal and the two resulting 

 cells not only differ markedly in size, but have a wholly different 

 prospective morphological value. The larger blastomere gives 

 rise to the entire mesoblast, to the germ-cells, the head-kidneys, 

 the ventral nerve-cord and the setae ; the smaller produces only 

 a portion of the head and of the alimentary epithelium. Let us 

 contrast Amphioxus and Nereis in this regard. In Amphioxus, 

 as in the echinoderm, differentiation advances very slowly. Not 

 until the actual invagination takes place can the limit between 

 the primary germ-layers be fixed, and the mesoblast arises still 

 later. In Nereis the median plane is marked out at the second 

 cleavage ; at the third the entire ectoblast of the trochal and 

 prae-trochal regions is formed ; at the fourth cleavage the 

 material for the entire "ventral plate" (including the ventral 

 nerve-cord and the seta-sacs) is segregated in a single cell, that 

 for the stomodaeum in three cells ; the fifth cleavage completes 

 the ectoblast, and by the 38-celled stage the germ-layers are 

 completely segregated (the mesoblast in a single cell) and the 

 architecture of the embryo is fully outlined in the arrangement 

 of the parent blastomeres, or protoblasts. 



In such a case as Nereis we cannot accept Oscar Hertwig's 

 implication that the relation between the individual blastomeres 



artificial separatioti. It would be very interesting to compare the cleavage of a 

 blastomere isolated immediately upon completion of the first cleavage with that 

 of one isolated just before the second cleavage. Much light might thus be thrown 

 on the general question of cellular interaction. 



