6o Edmund B. JVilson. 



rings, the resemblance to these relations of those observed in 

 Dentaliiim is unmistakably obvious. It is entirely possible that 

 the correspondence is not complete; but that in a general way 

 the resemblance indicates a similar form of stratification in the 

 molluscan and annelidan egg, seems hardly open to question; and 

 the comparison is sustained by the fact that in Clepsine both rings 

 were traced by Whitman into the AB half, and the upper one 

 into the D quadrant, while in Rhyfichelviis Vejdovsky traced both 

 rings into the D quadrant, where the material of the two fuses into 

 one mass in the 4-cell stage and later passes into the mesomeres, 

 which are undoubtedly to be identified with the somatoblasts.^ 

 If this comparison be admitted a further comparison of these 

 and some other forms is highly significant. In Dentaliiim three 

 structural zones are present from the beginning, the lower one 

 coinciding in extent with the lower white area, the upper one 

 lying at the centre of the upper white area, at first very small, but 

 rapidly increasing in extent during and after the maturation 

 period. A condition similar to this exists in Sternaspis, where 

 Vejdovsky ('81) showed that a distinct protoplasmic area, which 

 he compares to a polar ring ('88, p. 122) lies at each pole of 

 the ovarian egg, the upper one being much smaller than the lower 

 one, though larger than in Dentaliiim. . In Clepsine and Rhyn- 

 chelmis three structural zones are likewise present, but tJiese first 

 appear during the maturation period with the development of the 

 polar rings, like the three zones described by Boveri ('01) in the 

 Strongylocentrotus egg. The egg of Myzostoma occupies, at 

 least in some respects, an intermediate position. No upper pro- 

 toplasmic disc has here been observed as yet, but the lower proto- 

 plasmic area is obviously represented by the green mass, which, as 

 Driesch ('96) has shown passes into the polar lobe, and subse- 

 quently certainly in part into the first somatoblast, and probably 

 in part into the second somatoblast, precisely as in Dentaliiim. 

 The interest of this case, compared with the foregoing, lies in the 

 fact observed by Driesch (which I can confirm) that before ma- 



1 Cf. Vejdovsky and Mrazek ('03, p. 454) ; see also the highly interesting 

 statement (p. 534) that the dense protoplasm of the polar rings ("Polplasmen") 

 can be recognized as such "in den Zellen des Mesoblasts inbesondere in den 

 grossen Mesomeren." 



