No. 3.] THE CELL-LINEAGE OF NEREIS. 367 



rectly, regards it as moulded upon the Coelenterate type, but 

 modified by secondary changes involved in the differentiation of 

 the mesoblast as a distinct germ-layer. 



In what direction may we seek to break away from this dead- 

 lock of opinion 1 It appears to me that the only course open 

 to embryological investigation is to examine more precisely the 

 origin of the gastrula itself ; to take as a starting-point not the 

 two-layered gastrnla, but the ovjnn. The "gastrula" cannot be 

 taken as a starting-point for the investigation of comparative 

 organogeny unless we are certain that the two layers are every- 

 where homologous. Simply to assume this homology is simply 

 to beg the question. The relationship of tJie inner and outer 

 layers in the various forms of gastrulas must be investigated not 

 only by detei^minijig their relationship to the adult body, but also 

 by tracing out the cell-lineage or cytogeny of the individual blas- 

 tomeres from the beginning of development ; and I am convinced 

 that many apparent contradictions that appear under the ordi- 

 nary germ-layer theory will disappear when thus examined. 



It is from this point of view that I have considered the devel- 

 opment of Nereis. It will be shown that many important organs 

 and systems of the annelid body can be traced back to parent 

 blastomeres — I propose to call them protoblasts (" Ur-zellen ") 

 — that are differentiated long before the completion of the " gas- 

 trula" stage, and whose relations to one another can be deter- 

 mined with all possible accuracy. The development in fact 

 suggests a mosaic (to use a comparison of Roux's) ; the ontog- 

 eny may be resolved into a series of individual organogenies, 

 each of which takes its beginning in a single protoblast or a 

 small group of them. I shall endeavor, therefore, first to deter- 

 mine the origin and mutual relationships of the individual proto- 

 blasts in the cleavage process, and, second, to trace their later 

 history. The relation of the protoblasts to the germ-layers of 

 the gastrula becomes afterwards a simple question. 



It remains to be seen whether this method of study will be 

 found sufficiently practicable in other animals to afford a satis- 

 factory basis of comparison, and to what conclusions it may lead, 

 but some interesting results have already been attained in this 

 direction. These researches need not be fully reviewed at this 

 point, but I must refer briefly to Whitman's epoch-making 

 researches on the Hirudinea,^ which not only opened a new 



1 Nos. 27, 28. 



