MATURATION PHENOMENA OF (iERM CELLS. 153 



esis. But in concluding this these writers do not give a satisfac- 

 tory explanation either why or how univalent chromosomes unite 

 to form bivalent ones, and so give no clue to a reason for the 

 chromosomes being paired in the fertilized egg. This point can 

 be settled only by a careful reexamination of the changes in the 

 early growth period, and here I shall simply call attention to cer- 

 tain appearances that speak for the first maturation mitosis in 

 Ascans being a reduction division. 



In the fertilized egg are two pairs of chromosomes, A, a and 

 B, b (Figs. 2830), one pair being considerably larger than the 

 other, and the two composing a pair sometimes differing some- 

 what in length bat being approximately equal in volume. As 

 Van Beneden first showed, two of these chromosomes come from 

 the ovotid, and two from the spermatozoon. Fig. 27 shows a 

 slightly earlier stage, the chromosomes in two groups, one group 

 derived from the male pronucleus and the other from the female 

 pronucleus. In the group to the right is a large and a small 

 chromosome (A, B} ; in the group to the left also a large and 

 small one (a, &). But A corresponds approximately to a in vol- 

 ume, and B to b. Therefore we may say that of the four chro- 

 mosomes in the fertilized egg (Figs. 27-30) a small paternal one 

 (from the male pronucleus) corresponds in volume to a small 

 maternal one (from the female pronucleus), and a large paternal 

 one to a large maternal one. In other words, of each pair of 

 chromosomes in the fertilized egg, one chromosome is paternal 

 and one maternal. Which two come from the male pronucleus, 

 and which two from the female pronucleus, there is as yet no. 

 means of deciding, for the two pronuclei appear structurally 

 alike. But to make my argument clear I will assume that A and 

 B are paternal, and a and b maternal. 



Now in the formation of the first polar body (Figs. 24, 25) we 

 find the two well-known quadripartite chromosomes. There are 

 two, not four; hence they are bivalent with regard to the normal 

 number. In each bivalent chromosome (tetrad) we should ex- 

 pect then to find two univalent chromosomes. Both these figures 

 (24, 25) were drawn with great care to get the exact proportions 

 of the parts of the chromosomes ; both represent the stage 

 where one plate of chromosomes is passing into the polar body. 



