4 Edmund B. JVilson. 



prelocalization exists In the egg at the time It leaves the ovary, 

 and probably much earlier, and long before even the Initial 

 stages of maturation and fertilization. Nevertheless, progressive 

 changes take place during and subsequent to maturation, which, 

 when compared with those occurring In other forms, show this 

 egg, as I believe, to be only the extreme of a series that connects 

 it with such forms as the nemertlne or echlnoderm, and brings 

 them under one point of view. 



The present paper deals mainly with the development of frag- 

 ments of the unfertilized egg of DentaUum, the eggs being cut 

 singly with the scalpel under the microscope and subsequently 

 fertilized, following the method of Delage ('99). I shall here 

 consider the development of isolated blastomeres only incidentally 

 for the sake of comparison, reserving a fuller account for a second 

 paper. It may be stated here, however, that the experiments on 

 this part of the subject demonstrate, even more conclusively than 

 do those of FIschel for the ctenophore-egg, that the cleavage of 

 the ovum, In both DentaUum and Patella, Is In fact what the 

 normal cell-lineage so clearly indicates, essentially a mosaic-work, 

 In accordance with Crampton's earlier experiments on Ilyanassa. 

 Blastomeres Isolated at any stage from the 2-cell onward con- 

 tinue to segment as if still forming part of a complete embryo; 

 and apart from the changes due to shifting of the cells, which, as 

 in the ctenophore, often lead to the displacements of the larval 

 structures and to the closing of the partial embryos, undergo 

 essentially the same differentiation as If united to their fellows. 

 Thus, the first two blastomeres, upon separation, give rise to two 

 dissimilar larvae, each of which Is defective and represents es- 

 sentially the same structures as would have been produced had the 

 two cells remained united; In hke manner, of the isolated cells 

 of the 4-cell stage, the larva from the D-quadrant possesses cer- 

 tain structures that are lacking in the other three; and the dif- 

 ferences among the larvae from cells of the 8- or i6-cell stages are 

 still greater. Cells procured by successive Isolations up to the 

 64-cell stage, or later, differentiate singly, according to their na- 

 ture, into actively swimming trochoblasts of three kinds; into 

 ordinary ectoblast- or entoblast-cells, into sensory cells bearing 



