Il6 CLEAVAGE AND DIFFERENTIATION 



the apical sense-organ at one end, and powerfully ciliated cells 

 characteristic of the prototroch at the other : these are separated by 

 non-ciliated epidermal cells. The types of cell and the number of 

 each type produced by the isolated i/8 micromere are the same 

 as it would have produced in the young swimming trochophore 

 larva if it had been left in place in the developing egg (fig. 55). 



Descendants of a 1/8 micromere, if isolated later, continue to 

 divide just as often and to produce just the same kind and number 

 of cells as would have happened in the whole intact embryo. For 

 instance, the vegetative member of the first product of division of 

 a micromere of the first quartet (i «2 to i d.^ in normal development 

 produces four ciliated prototroch cells. It does the same if isolated ; 

 while either of the products of its division divides once only to 

 produce two ciliated cells. 



Isolated cells of the second quartet {2 a to zd) produce certain 

 ciliated cells which contribute to the prototroch, certain others of 

 a different type which belong to the pre-anal ciliated band, non- 

 ciliated epidermal cells, and larval mesenchyme in the interior. 

 These types of cells in the same numbers are produced by the 

 micromeres of the second quartet in normal development. 



The exact meaning of these facts has not been determined. Pre- 

 sumably, two agencies are at work. First, certain chemo-differ- 

 entiated substances are probably restricted to particular micro- 

 meres ; secondly, it appears that the number of cleavage divisions 

 possible to any isolated blastomere is fixed. This may perhaps be 

 correlated with the fact (described below, see p. 132) that nuclear 

 synthesis during cleavage takes place at the expense of certain 

 materials in the cytoplasm, present in finite amount. When 

 these materials are exhausted in an isolated cell (deprived of 

 contact with the yolk of its Qgg^ or other food-supply), cleavage 

 stops. 



It is probable that in all Annelids and Molluscs (other than 

 Cephalopods), even when no differentiated substances can be de- 

 tected in the uncleaved egg, they do in fact exist, and are distributed 

 during cleavage in a similar way. Only on these lines can such facts 

 be explained as the almost universal restriction of the potentiality 

 of forming mesoderm bands to 4^, and of forming ectoderm bands 

 to zd. The very general fact that the Z) 1/4 blastomere is larger than 



