Experimental Studies on Germinal Localization. 59 



that the contrast between the development of an egg-fragment in 

 the nemertine and in a sea-urchin (where it segments like a whole 

 egg only after section in certain planes) is owing to the fact that 

 in the latter, egg-fragments have only been obtained in the period 

 subsequent to maturation when the regrouping has been effected. 

 Localization of the cleavage-factors was thus conceived, essen- 

 tially in agreement with Roux's early conclusions regarding the 

 frog's egg, as a progressive (i. e., epigenetic) process, and the 

 same conception was applied to the general morphogenic process 

 which, as is shown with especial clearness by the facts here brought 

 forward, may be so closely connected with the cleavage-process. 



As far as the progressive character of localization is con- 

 cerned, the result obtained in Dentalium may seem at first sight 

 to be in disagreement with the conclusions just reviewed, for the 

 germ-regions are here defined by a definite segregation of ma- 

 terials that exists even in the attached ovarian egg long before 

 either maturation or fertilization, and the isolated blastomere 

 is not capable of producing a complete embryo. But the contra- 

 diction disappears upon comparison with certain other forms, 

 which are intermediate in character between the extremes repre- 

 sented by Dentalium and the nemertine or echinoderm egg; and 

 this comparison demonstrates, as I believe, the validity of the 

 theory of "precocious segregation," formulated as a pure specula- 

 tion by Ray Lankester in 1877. I have already expressed the 

 opinion that the horizontal stratification of the egg expressed by 

 the three zones of material visible in Dentalium or Myzostoma is 

 comparable, or at least analogous, to that which finds an expres- 

 sion in the formation of the well-known polar rings of leeches and 

 oligochaetes. This comparison is based both on the position and 

 mode of formation of these rings and on their fate. Vejdovsky 

 ('88) very clearly shows that in Rhynchelmis both the polar rings 

 arise as local thickenings of a general ectoplasmic layer, and both 

 assume at one period the form of protoplasmic discs lying at 

 either pole of the egg (as Whitman also observed in Clepsine) . 

 Except for the fact that the upper and lower protoplasmic areas 

 have not at any period been seen to appear in the form of actual 



