58 Edmund B. Wilson. 



On this point an analogous result has recently been obtained by 

 Yatsu, who has shown with great probability that in the unseg- 

 mented nemertine egg the basis of the apical organ does not lie 

 at the upper pole, where we should expect to find it, but in, or 

 slightly above, the equatorial region. 



These facts have an important bearing on our interpretation of 

 development in general. In my previous paper on the nemertine 

 egg I have developed an hypothesis of differentiation agreeing 

 broadly with Sach's well-known theory of formative stuffs, and 

 with the general conclusions regarding mosaic development inde- 

 pendently published by Fischel ('03) nearly at the same time, the 

 essential assumptions being that the prospective value of a cell is de- 

 termined by its cyVoplasmic content, that this content is de- 

 termined by the form of cleavage in connection with an antecedent 

 formation and segregation of specifically different materials 

 (which may Itself determine the form of cleavage), and that 

 the morphogenic function of cleavage, so to say, is to isolate 

 the materials thus segregated. This conception, it is hardly neces- 

 sary to point out, receives very definite support by the observations 

 now brought forward; but I wish to bring them more closely into 

 relation with those made on the nemertine and echinoderm eggs, 

 especially with regard to the general question of progressive (i. e., 

 epigenetic) localization in the egg. In the nemertine {Cerebra- 

 tulus) I found that either an isolated blastomere or a fragment 

 from any region of the unsegmented egg may produce a perfect 

 dwarf larva; but the two differ In the form of cleavage, the 

 blastomere segmenting as if still forming part of a whole em- 

 bryo and producing an open blastula (as in the echinoderm), 

 while the egg-fragment segments like a whole egg and produces 

 a closed blastula — that is, it develops as a whole from the be- 

 ginning. I explained the contrast In development between the 

 two as the result of a regrouping of the egg-materials, occurring 

 during and subsequent to the process of maturation and fertiliza- 

 tion, which Initiates the morphogenic process and determines also 

 the form of the earlier cleavages. I pointed out that such re- 

 grouping of materials is known to occur at the maturation-period 

 of many eggs — for Instance, in the sea-urchin — and suggested 



