LINVILLE : PULMONATE GASTEROPODS. 231 



those in the group that is to go into the first polar cell and those that 

 are to remain in the egg, have this dumb-bell shape. Even in the meta- 

 kinesis of the first maturation division I have occasionally seen evideuce 

 of a transverse constriction of the chromatin rods. Thus we seem to 

 have in Limnsea a partially concealed " tetrad formation." The presence 

 of the transverse constriction so soon after the longitudinal splitting 

 leaves little doubt that the division as it finally takes place in the second 

 maturation spindle agrees in position with the early constriction. 



This observation does not accord with the results Boveri ('90) ob- 

 tained for Carinaria, one of the heteropods. He found the chromosomes 

 of the first maturation spindle to be quadruple. This quadruple group 

 splits longitudinally in the first maturation divisiou, and then, after the 

 rearrangement of the chromosomes on the second maturation spindle, 

 the division takes place by a longitudinal splitting, exactly as in the 

 first maturation spindle. ^Naturally one is disposed to ask, What, then, 

 is the meaning of the quadruple groups'? 



The thing of importance now to be decided for Limnsea is the manner 

 of formation of these " Vierergruppen " or tetrads. To make sure of 

 an exact answer to that question a study of the processes going on in 

 the rearrangement of the chromatin in the germinative vesicle in pre- 

 paration for the maturation divisions would be necessary. If the tetrads 

 are formed by two longitudinal splittings of segments of the original 

 spireme thread, as in Ascaris (Boveri, '87), and if the two pairs of ele- 

 ments composing the tetrad are elongated and pressed closely together, 

 as at least they seem to have been in the case of Cafinafia, then the 

 two maturation divisions would be " equation divisions," and not re- 

 ducing, except in the sense of a quantitive reduction. If, on the other 

 hand, the length of the masked tetrads represents the length of the 

 original spireme which, in breaking up, first divides longitudinally and 

 next transversely, then the second maturation division is a reduction 

 division in the Roux-Weismann sense. Apart from the evidence which 

 a study of the early stages in the formation and fission of the spireme 

 thread would give, I have little hesitation in holding that we have in 

 Limnoea a reduction division of the Roux-Weismann type, because soon 

 after the evident longitudinal splitting of the chromosomes there occurs 

 a transverse constriction in each of the resulting halves, which continues 

 until there is complete separation of each half into two parts, which 

 then move toward their respective poles. 



