260 BIOLOGICAL LECTURES. 



of cleavage, it is very difficult for me, at least, to believe that 

 it occurs, especially as I cannot see that there is evidence to 

 warrant the conclusion. There are, of course, cases where cer- 

 tain substances have been found to pass always into definite 

 cells, e.g., the granules of the " Urvelarzellen " of Neritina 

 (Blochmann, ( 8l) and certain granules in Asplanchna (Jennings, 

 '96). Evidence is lacking, however, to show that these sub- 

 stances are necessary to the formation of the organs to which 

 the cells give rise. Conklin ('99, p. 18) admits that there is 

 abundant evidence to prove the absence of any necessary rela- 

 tion between cell formation and differentiation, but believes 

 that in certain cases the two processes are related. He distin- 

 guishes three categories of relation : (i) " Cell formation follow- 

 ing the lines of preceding differentiation, e.g., certain cleavages 

 of ctenophores, mollusks, and ascidians ; or (2) cell formation 

 and concomitant differentiation, e.g., many cleavages of turbel- 

 larians, nematodes, annelids, and mollusks ; or (3) differentia- 

 tion following the lines of preceding cell formation, e.g., many 

 cleavages in the eggs of annelids, mollusks, and probably many 

 other animals." The significance of these so-called relations 

 becomes apparent when we observe that every differentiation 

 which occurs in multicellular material must be related in one 

 of these three ways to cell division. In other words, cell divi- 

 sion and differentiation are just as independent of each other 

 here as elsewhere, but the process of differentiation is occur- 

 ring to a certain extent during a series of very conspicuous cell 

 divisions, i.e., the earlier ones. A comparison of the process 

 of formation of a single organ, the prototroch, in different ani- 

 mals seems to me to show how absolutely distinct cell formation 

 and differentiation are. The prototroch may consist of eight 

 or of sixteen cells, of twenty-five, of thirty, or of many ; the 

 series of divisions leading to its formation are almost always 

 different in different forms, and where they are not they are 

 similar simply because they are purely spiral cleavages ; it may 

 become functional almost immediately after the divisions are 

 completed, indeed in Capitella (Eisig, '98) divisions continue 

 after ciliation, or it may remain a long time before becoming 

 ciliated ; and, finally, cells perfectly equivalent to the " primary 



