Artificial Parthenogenesis m Thalassema MeJlita 123 



during the anaphase the halves of the dyads move to the poles 

 (Fig. 23). A comparison of Figs. 21 to 23 w^ith Griffin's Figs. 18, 

 21, 25, will indicate their close identity. The second polar body 

 is constricted off in the same manner as the first and con- 

 tains twelve single chromosomes, while the same number is found 

 in the egg. I have occasionally observed a doubling of the 

 inner centrosome of the second spindle, as described by Griffin; 

 Fig. 23 shows a case in point. The twelve chromosomes left in 

 the egg (Fig. 24) pass at once into vesicles which at first only par- 

 tially fuse to form the egg nucleus, while every trace of the inner 

 centrosome and its accompanying rays soon disappears (Fig. 25). 



As described by Conn ('86) and by Griffin ('99), the first polar 

 body invariably divides, and, as the latter has determined, it does 

 so by a complete mitosis. The same division has been repeatedly 

 observed in the parthenogenetic eggs, as shown in Fig. 4, which 

 is drawn from a living egg, and also in Figs. 21, 23, 25 and 45 

 where the minute spindles and rudimentary asters with their 

 centrosomes are clearly seen in section. 



In the fertilized egg, the second polar body never exhibits an 

 attempt at division (Griffin, p. 615), but observations on living 

 eggs, which have been exposed to acid solutions, show that all 

 three polar bodies continue to divide until a cluster of miniature 

 cells have been formed, as already described (Fig. i). Although 

 I cannot state that all of these divisions take place mitotically, I 

 have repeatedly observed distinct indications of spindles in sec- 

 tions of the groups of polar cells; Fig. 62 shows, for example, a 

 spindle in two of the five cells present in the section. 



Up to this point the history of the parthenogenetic egg is identi- 

 cal with that of the egg fertilized by sperm, except for the absence 

 in the former of the spermatozoon and its aster, and all of the 

 details of the process of rpaturation which Griffin has so carefully 

 worked out may be easily verified on the eggs of my experiments. 

 From now on, however, a special description of the changes taking 

 place in the parthenogenetic eggs becomes necessary, for the 

 absence of the sperm nucleus and asters radically alters certain 

 subsequent events, and a special account, quite different from that 

 of the usual phenomena, must be given of the origin of the cleavage 

 nucleus and amphiaster. 



