1 6 LILLIE. [Vol. X. 



fourth product determined whether or not it is to be meso- 

 blast ; but that this conclusion is not justified is shown by the 

 fact (which Mr. Crampton very kindly permits me to add) that the 

 macromere which is to form the mesoblast divides before the 

 other. Its prospective function is structurally outlined before 

 this cleavage. For the origin of this reversed mode of cleav- 

 age we must go back to the two-cell stage. The cleavage 

 from two to four cells is leiotropic in Limnaea, and dexiotropic 

 in Physa ; on this depends the localization of the mesoblast 

 in the first case in the left, in the second in the right posterior 

 macromere. Some structural difference of the two ova con- 

 ditions the primary difference in the direction of the cleavage, 

 on which all subsequent variations depend. 



This is the first time,i as already stated (No. 21), that an 

 eight-cell stage has been found to exist in the lamellibranchs 

 resembling the form so characteristic for this stage in all other 

 Mollusca (Dentalium excepted), with holoblastic ova, as well 

 as in Annelida and Polyclada, not to mention numerous other 

 forms. This stage, in its typical form, consists of four micro- 

 meres lying upon and alternating with four macromeres, the 

 former being derived one from each of the latter, and lying 

 to the right of the parent macromere. It is true that Flem- 

 ming describes the origin of the eight-cell stage in Anodonta 

 in the same way as I have done, but he does not figure it 

 clearly. He says (p. 130, l.c): " Nachdem der Keim in dem 

 Stadium der Fig. 10" (four-cell stage) "wieder langer minde- 

 stens mehrere Stunden geruht hat, beginnt der nachste Act 

 der Theilungsarbeit, und zwar wieder in analogen Weise wie 

 der letzte : auch jetzt theilt sich der dunkle Obertheil (i.e., D) 

 in zwei ungleich grosse Segmente, andererseits proliferirt der 

 jetzt dreizellige Untertheil: nur laufen diese Processe hier 



1 Korschelt and Heider state in their " Lehrbuch " that Lankester described 

 for Pisidium an eight-cell stage formed of two superimposed layers of four cells 

 each, and they refer the reader to his well-known article (No. 54). Any one who 

 will take the trouble to read the first two pages of his work and to look at his 

 P'ig. 17, will find that Lankester described four meridional furrows before the 

 appearance of the " first circumferential," or equatorial furrow. Thus the eight- 

 cell stage would consist of eight cells in one plane. This is undoubtedly an error 

 of observation. It is, however, different from the account of Korschelt and 

 Heider. 



