Experimental Studies on Germinal Localization. 249 



these facts to those determined in the nemertine and mollusk is 

 considered beyond. I will at this point only express my agree- 

 ment with the conclusion of Fischel, that "Sowohl bei den Echin- 

 odermen, wie bei den speciell so-genannten 'Mosaik-eier' erfolgt 

 die normale Entwickelung im Wesentlichen als Mosaik-arbeit" 

 (Fischel, '03, p. 728). I am convinced 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 hy- 

 pothesis of differentiation, which proved to be untenable, the 

 whole discussion would have taken a very 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. 



My experiments on the unsegmented egg of Dentalinm have 

 added fresh proof to that obtained by Fischel and his predeces- 

 sors in the ctenophore, that the cleavage-mosaic is a mosaic of 

 specifically different cytoplasmic materials, in which are somehow 

 involved corresponding morphogenic factors. In this egg, con- 

 firming and extending the earlier work of Crampton on the gas- 

 teropod-egg, I was able to show even more definitely than has been 

 done in the ctenophore-egg the existence in the unsegmented egg 

 of prelocalized cytoplasmic regions, distinguishable by the eye, 

 that stand in some necessary relation to the formation of the 

 structures to which they give rise in the normal development; for 

 if one of these areas (the lower polar area) be removed, the 

 structures to which it is destined to give rise fail to develop, while 

 if this area remains while other areas are removed the structures 

 in question make their appearance. His's principle of "Organ- 

 bildende Keimbezirke," which he developed in a purely descrip- 

 tive sense, is thus shown to have a true causal significance. Since, 

 further, this area contains no nucleus, the conclusion is unavoid- 

 able that here, as in the ctenophore — and as we are now able to 

 say, even in the echinoderm — there is a localized distribution to 

 some extent, of the factors both of cleavage and of differentia- 

 tion in the cytoplasm before development begins. A no less sig- 



