500 The Ohio Naturalist. [Vol. IX, No. 7, 



THE FIRST NUCLEAR DIVISION OF THE MOTHER-CELL. 



All sixteen chromosomes divide almost simultaneously. To 

 determine the plane of this division was no easy task. That it 

 is a transverse division is quite evident from the cells examined. 

 Figs. 14a, 16a, 16b, show some chromosomes dividing trans- 

 versely, and since this is the only transverse division, it should 

 be considered qualitative. The spindle fibers end near or at the 

 free ends of the chromosomes. The homologous daughter 

 chromosomes show not only a marked correspondence in size, 

 but also in shape (Fig. 14, 14a, 15, 16b). Whether this similaritv 

 in shape is inherent, or is the result of a stress brought about by 

 the spindle fibers, or is the result of a repulsion of the daughter 

 chromosomes has not been determined. J, I, V, U, shaped 

 chromosomes are present. I and J shaped predominate (Figs. 

 15, 16). In rare instances indications of a median cleft through- 

 out the length of the chromosome in metaphase could be noticed 

 (Fig. 14a). This is probably the beginning of the second 

 division. Since the daughter chromosomes show such a marked 

 similarity in shape, the author was at first skeptical about the 

 plane of the division, but enough data are now at hand to prove 

 that the division is transverse. Figs. 14a, 14b, 15a, 16a, 16b, 

 etc., show this. In the migration of the daughter chromosomes 

 to their respective poles, a doubleness is occasionally detectible 

 (Figs. 16, 17a). This is probably the beginning of the second 

 division. A wall now develops and a distinct nuclear plate is 

 seen between the daughter nuclei (Fig. 17). The micronuclei 

 apparently re-enter the nucleus (Figs. 17, 18). 



, THE SECOND NUCLEAR DIVISION OF THE MOTHER-CELL. 



The daughter nuclei do not enter into a definite period of rest, 

 and the chromosomes soon become developed into the mother 

 skein of the second division. The transition is so rapid, and the 

 telophase of the first division and the prophase of the second are 

 so close together, that the individual chromosomes seem to be 

 separate from the beginning. Occasionally, a cell is seen in which 

 the second division has already taken place, while its sister cell 

 is still undivided (Fig. 22). Ordinarily, the sixteen chromo- 

 somes of each of the two daughter cells of the first division are 

 almost immediately ready for the second division. Sometimes a 

 cell is seen in which some chromosomes, the double nature of 

 which is apparent, lie in a horizontal position in prophase, while 

 other chromosomes, which are slightly more advanced, are at 

 right angles to the horizontal ones, and are already' migrating to 

 their respective poles (Fig. 19). There seems to be little prob- 

 ability that this is a cell in which some chromosomes are dividing 

 longitudinally while others are dix'iding transverse!}' as McClung 



