2l6 EJu'in G. Conklin. 



the later stages," I find that in the ascidian egg all the principal 

 organs of the larva are represented by distinct organ-forming 

 substances which are localized in their definitive positions and 

 proportions as early as the close of the first cleavage. 



There is a world-wide difference between such results as these 

 and those which were reached by Driesch and some of the earlier 

 workers in this field. Wilson ('04) has recently expressed the 

 opinion that *'had the experimental analysis of cleavage been first 

 undertaken in the case of such a determinate type as that of the 

 gasteropod or annelid and had Roux not handicapped his theory 

 with a purely speculative hypothesis of differentiation, which 

 proved to be untenable, the whole discussion would have taken a 

 different course; and I believe it would from the first have been 

 recognized that the mosaic principle holds true in greater or less 

 degree for every type of development, not excepting the most 

 'indeterminate' forms of cleavage." Considering the fact that 

 such highly determinate forms as the ascidian and the cteno- 

 phore were studied in some of the earliest experiments on the 

 potency of cleavage cells, I am of the opinion that the course 

 which this discussion took was not primarily due to the fact that 

 work began on relatively indeterminate forms. On the. other hand 

 I am convinced that the whole trend of opinion on the organization 

 of the egg and on the potency of cleavage cells would have been 

 different if those who did this work had been more familiar with 

 the normal development of the forms studied, and in their zeal for 

 the experimental method had not discarded the old and approved 

 method of observation. It has taken such careful observers of 

 normal processes as Boveri and Wilson to apply most successfully 

 the experimental method to the problem of the organization of the 

 egg, and the results of such work constitute a well-deserved tribute 

 to the permanent value of the work of Roux. 



SUMMARY. 



I. Normal Development. 



I. In the ovarian egg of Cynthia (Styela) partita there are three 

 strikingly different kinds of ooplasm, viz: a superficial vellow layer, a 

 central gray area, and a large transparent germinal vesicle. At 

 this stage the localization of these substances is approximately 

 concentric. 



