THE SPIRAL TYPE OF CLEAVAGE. 



259 



all other cells of the Qgg in size, as is the case in Arenicola. 

 It seems at least unnecessary to suppose, however, that this 

 cell contains an entirely different "organ-forming substance" 

 from that contained in, for instance, the four cells, products of 

 the dorsal intermediate girdle cell of the first quartette, which 

 pass through the dorsal gap in the prototroch, and lie just in 

 front of the somatic plate (Fig. 13), or from that contained 

 in the cells of the second and third quartettes which form the 

 ectodermal region about the mouth. Indeed, Dr. Treadwell 

 tells me that in Podarke a large portion of the dorsal ectoderm 

 of the trochophore is formed by the descendants of the dorsal 

 first quartette cell, which have passed through the gap in the 

 prototroch. 



Of course, difference in size between yolk-bearing and non- 

 yolk-bearing cells is an expression of difference in constitution. 



Notwithstanding the fact that the spiral forui of cleavage 

 remains throughout the earlier stages of development, a form 

 of modification does occur which in a way anticipates the morpho- 

 genetic character of the later cleavage. The determinate char- 

 acter of the morphogenetic period establishes the fate of each 

 cell under normal conditions ; and, moreover, it renders possible 

 a further degree of modification in the cleavage, viz., " preco- 

 cious segregation." As the result of precocious segregation, 

 the blastomeres not only have a definite fate, but each contains 

 a definite amount of material, according to the part which it is 

 to play in the cleavage and irrespective of the yolk which it 

 may contain. This modification extends back to the earliest 

 stages of cleavage, but without altering the spiral form, not- 

 withstanding the great differences in pressure that may exist 

 between the blastomeres. It has become the custom to speak 

 of a protoblast as containing from its earliest appearance the 

 substance for the organs which are to be formed by its descend- 

 ants, and the term "differential cleavage" has been employed 

 to denote the divisions which give rise to the protoblasts. Pre- 

 cocious segregation is undoubtedly connected with differentia- 

 tion in so far as the perfectly exact distribution of the material 

 renders an earlier differentiation possible ; but, as regards the 

 separation of various organ-forming substances by the planes 



