Experimental Studies on Germinal Localization. 6 1 



turation the egg shows at first but two colored zones, of which 

 the lower green one exactly represents the lower white area of 

 Dentalium, while the upper one first segregates during maturation 

 into an upper red zone and an equatorial colorless one. Like 

 the lower zone the two upper ones correspond very closely in fate 

 to those in Dentalium; for the upper (red) area passes into the ec- 

 tomeres, like the upper white area of Dentalium, while the middle 

 (colorless) zone passes into the entomeres, as is the case with the 

 greater part of the middle (pigmented) zone in Dentalium. It is 

 possible that sufficiently careful search may reveal the presence 

 in Myzostoma of an upper protoplasmic disc, comparable with 

 a polar ring; and as far as the visible colored zones are concerned, 

 it is evident that the Myzostoma egg stands midway between those 

 of Dentalium and Strongylocentrotus , and it is probably inter- 

 mediate also between Dentalium or Sternaspis and Clepsine or 

 Rhynchelmis. 



It seems a legitimate interpretation of the foregoing series 

 that these eggs present an essentially similar form of stratification 

 which is attained at different periods in the ontogeny, and that 

 as compared with the leech or oligochaete, Myzostoma and Den- 

 talium or Sternaspis represent two earlier stages in the precocious 

 segregation of specific cytoplasmic materials that have a like pros- 

 pective value in the development.^ But if this be admitted, it 

 follows that in none of these cases can the segregation in question 

 be considered as a primary character or "preformed quality" of 

 the^egg. .Upon this secondary localization of material, as my 

 experiments prove, depend many of the most important features 

 of the later morphogenic localization; and I think a presumption 

 Is thus established that cytoplasmic prelocallzation is in general of 

 like secondary or epigenetic origin, though to what extent this 

 holds true can only be determined by further experiment. 



Although the characteristic segregation is In its main outlines 

 effected very early In the egg of Dentalium, It may be pointed out 

 that, like so many other eggs, there is the clearest evidence of 



1 Cf. Vejdovsky "Wahrend aber bei Sternaspis die Concentration des Bil- 

 dungsplasma an beiden Polen bereits im Laufe der Eibildung stattfindet, sam- 

 melt sich dasselbe bei Rhynchelmis erst nach der Polzellenbildung und dem Ein- 

 dringen des Spermatozoon in das Ei an" ('88, p. 122.) 



