STUDIES ON CHROMOSOMES 419 



a real exception to the rule; for such 'tetrads' obviously differ 

 entirely from the true tetrads of the maturation-processes, which 

 represent synaptic pairs of the diploid groups. Perhaps an analo- 

 gous case is that of Cyclops and Diaptomus, where Haecker ('95, 

 '02) long since demonstrated a cross-bar or suture in each com- 

 ponent of each bivalent; and since each of these component, is 

 sooner or later longitudinally divided, double tetrads or 'ditet- 

 rads' are thus produced. The work of Braun ('09) and Matschek 

 ('09, '10) confirms this but, like that of Haecker himself, proves 

 that the cross suture is without significance for the divisions, 

 since both the latter are longitudinal (cf. also Lerat, '05); while 

 Krimmel ('10) has shown that in Diaptomus the transverse con- 

 striction or suture is also present in the univalent chromosomes of 

 the diploid groups in somatic cells. Haecker's conclusion that the 

 cross-suture represents the point of end to end synapsis is a quite 

 unproved, and I think very improbable, assumption. In view of 

 what is seen in Syromastes, Notonecta, Phylloxera and Nezara, 

 it seems more likely that each univalent chromosome in these 

 forms consists of two closely united components, of which the 

 cross-suture is an expression. It seems quite possible that the 

 number of chromosomes might change in these animals by com- 

 plete separation of the two components in case of one or more of 

 the chromosomes. 



Keeping in view all such apparent exceptions, the fact remains 

 that the heterotypic (or tetrad) form is a highly characteristic 

 feature of the maturation prophase-bivalents, and that in this 

 respect they show in general a marked contrast to the univalents, 

 here and elsewhere. The meaning of this in case of the matura- 

 tion-divisions is unmistakable; and the burden of proof may be 

 left with those whose theoretical prepossession will not allow them 

 to accept the natural explanation that here is manifest. 



3. The chromosomes and heredity 



That the chromatin-substance (more specifically that of the 

 chromosomes) plays some definite role in determination has 



