CHANGES IN REPRODUCTIVE CELLS OF ELASMOBRANCHS. 3801 
hereditary substance, argues, like Weismann, for the necessity 
of some sort of chromatic reduction, before the maturation of 
sexual cells; but he comes also to the conclusion that the 
reduction processes hitherto described are numerical reductions 
of the chromosomes, and not quantitative with respect to their 
substance. He draws a sharp distinction between “id”? re- 
duction and chromosome reduction, the latter of which he 
apparently disregards, but he seems to entertain the idea that 
the former, by the numerical reduction of the chromosomes, 
may in reality be carried out. He shows further that the 
numerical reduction of the chromosomes in ovogeneses, like 
that of Echinus and Pterotrachea for example, repre- 
sented in the diagram which I have borrowed (Diagram II), 
and which will become intelligible on reference to my § 44, is 
not brought about by any of the divisions in the first ovogenetic 
period, a, and up to the formation of the first ovocyte after the 
rest (3, i.e. the ovum before the extrusion of the polar bodies, 
but that there are only half as many chromosomes in the first 
ovocyte when it emerges from the rest, (3, in the polar body- 
spindle as there were before, and this number is retained after 
the polar bodies are extruded in the ovum. ‘Therefore the 
numerical reduction is not brought about by any division of 
the ovogenesis, but occurs during the synaptic rest, 3, and 
before the prophasis of the first “ Richtungspindel.”’ 
a p Y 
; ‘ LS SSS 
Ag)» 
4) No) 1(¥) 4 
Diagram II, representing course of typical ovogenesis (after Boveri). 
Reference letters same as in I. 
Finally, he shows that the so-called reduction processes 
hitherto described are irreconcilable among themselves, and 
