No. 3.] THE CELL-LINEAGE OF NEREIS. 455 



and median planes respectively. Among the annelids, Nereis 

 agrees with Crepidiila, while Clepsine (Whitman) and RJiyncJiel- 

 mis (Vejdovsky) agree with Planorbis, Neritina, and the poly- 

 clade. It is perfectly clear that this difference can have no 

 morphological significance, but is a result of mechanical causes 

 of some kind. Either the ectoblastic cap or the group of macro- 

 meres has been rotated 45° from its primary position, but I am 

 at present unable to suggest an explanation that will satisfy all 

 of the conditions of the problem. These differences certainly 

 afford a forcible illustration of the fact that the relations of 

 parts in the early stages of development may readily be modified 

 by secondary conditions. 



In conclusion, I may call attention to the bearing of my gen- 

 eral interpretation of the cleavage upon the significance of the 

 protoblasts and the study of cell-lineage. The general results 

 show that it is necessary to be very cautious in drawing mor- 

 phological conclusions from the comparative study of early 

 cleavage-stages. I cannot entirely agree with Watase that 

 " the earlier cleavage-processes are more fundamental, and, 

 from the morphological standpoint, more significant than the 

 later ones." Blastomeres having precisely the same mode of 

 origin and precisely the same spatial relations to the rest 

 of the embryo are by no means necessarily equivalent, either 

 physiologically or morphologically, and the early cleavage-stages 

 in themselves have little morphological value. The respective 

 values of the blastomeres must be determined by their ulti- 

 mate fate ; and this is an indispensable datum for the study of 

 comparative embryonic anatomy. TJie fitndaniental fonns of 

 cleavage are primarily due to mechanical conditions, and are only 

 significant morphologically in so far as tJiey have been secondarily 

 remodelled by processes of precocious segregation. To this preco- 

 cious segregation we must ascribe the early differentiation of 

 the protoblasts. The facts show, however, that this process 

 has very generally taken place in a greater or less degree ; and 

 that from the study of cell-lineage, if rightly applied, we may 

 hope ultimately to attain a firm basis for an estimate of the dif- 

 ferent forms of gastrulas and a comparison of the germ-layers. 



Munich, December, 1891. 



