406 BARBARA WIGGEXHORX AXD DAVID D. WHITNEY. 



the early stages of the embryo is not determined. Perhaps as 

 the cells become more numerous and consequently smaller the 

 size ratio between nucleus and cytoplasm is so altered that 

 there is only room for one mitotic figure and consequently both 

 nuclei are crowded into one fused nucleus. 



A section was found of a fertilized egg in the one-cell stage 

 in which the chromosomes could be counted fairly accurately. 

 Each nucleus seemed to have 26 chromosomes. This fact 

 clearly differentiates it from either of the parthenogenetic eggs. 

 It also furnishes additional evidence to support the claim that 

 the fertilized egg was originally the male parthenogenetic egg 

 which has been entered by the sperm. The haploid number of 

 26 chromosomes is found in the mature male parthenogenetic 

 egg and in each of the two nuclei of the fertilized egg. One of 

 these nuclei in the fertilized egg is undoubtedly the egg nucleus 

 and the other is from the sperm (Fig. 2, C}. 



Further observations of the sections of the fertilized eggs show 

 that in the first cleavage each nucleus gives rise to two groups 

 of chromosomes each of which passes separately to the daughter 

 nuclei (Fig. 2, D}. During the ensuing resting stage each germ 

 nucleus is represented by a structurally distinct vesicle. Thus 

 the separateness of the germ nuclei is maintained throughout the 

 entire nuclear cycle. In mitosis there seem to be two spindles 

 each with its distinct set of chromosomes which separate regard- 

 less of each other. Probably each group of chromosomes splits 

 into halves, and a maternal and a paternal group go to each 

 end of the double spindle, so that each daughter cell receives 

 two sets of chromosomes around which separate walls are formed, 

 and a maternal and a paternal vesicular nucleus appear. 



The double nuclei have been found up as far as about the 

 250-cell stage. In one section of an embryo there were 103 

 visible cells and among these two had double nuclei (Fig. i, 

 /I-2). 



Two distinct nuclei in fertilized eggs and their developing 

 embryos have been observed in other forms by various other 

 workers. Mark observed this phenomenon in Limax, Van 

 Beneden in Ascaris, Hacker in Cyclops, Conklin in Crepidula, 

 Beard in Raja batis, Smith in Cryptobranchns allegheniensis and 

 other workers have seen indications of it in other forms. No 



