46 The Chromatin in the Development of Hybrids 



and really represent the materna] and paternal chromatin, Conklin 

 expresses the situation in the following words : " It still remains to 

 show that these double nuclei really represent the egg and sperm nuclei 

 which have not yet lost their individuality. This cannot be demon- 

 strated in Crepidula, for the reason that this double character is not 

 apparent at every stage in the nuclear cycle, but it is extremely probable, 

 as the following observations will show : " The detailed reasons given 

 need not be repeated here. 



There are but two ways to demonstrate this with certainty, namely, 

 either to follow the process in the living egg, or to be able to distinguish 

 the two kinds of chromosomes, as I have been able to do in the hybrid 

 under consideration. 



Herla, 93, and Zoja, 95, made some observations which bear directly 

 upon this point. In the study of an Ascaris containing eggs in various 

 stages of early cleavage they found that the number of chromosomes in 

 the cells was only three, one of which was slightly smaller and like the 

 chromosomes of the variety univalens. The eggs, they conclude, were 

 hybridized by the sperm of univalens. They were able to trace the 

 independent maternal and paternal chromosomes to the 12-cell stage. 

 With only three chromosomes it is not possible to determine with certainty 

 very much about their distribution in the spindle. Zoja, in fact, says 

 that the small chromosome may vary its position with reference to the 

 other two, sometimes being between the two latter. These observations, 

 therefore, can throw little light on the particular question of the distri- 

 bution of the two chromatins in the nucleus. 



In my own hybrids, however, where the number of chromosomes is 

 great, any disturbance of their grouping can be readily made out. The 

 conditions described for these hybrids, taken in connection with the obser- 

 vations of Herla and Zoja, demonstrate in the clearest manner that the 

 two chromatins may remain distinct in these resting nuclei and that the 

 chromosomes in the subsequent division may be and are grouped spa- 

 tially. They, furthermore, lend the strongest support to the belief that 

 in the other forms described (Cyclops, Crepidula, Pinus) the two groups 

 of chromatin or chromosomes arising from the bilobed resting nuclei 

 of the two first blastomeres may really represent the distinct parental 

 chromosomes. 



7. The Rotation of the Cleavage Nuclei During the First two Cleav- 

 ages. — In the first cleavage spindle the chromosomes lie side by side in a 

 horizontal plane. In this same plane they can be followed into the early 

 resting stage of the two daughter nuclei. Evidently, inasmuch as the 

 second cleavage plane forms at riglit angles to the first, the nucleus will 



