142 The Ohio Naturalist. [Vol. XIII, No. 7, 



A CYTOLOGICAL LIFE CYCLE. 



Robert F. Griggs. 



The figures and diagrams which are usually presented to 

 explain the nature and significance of the reduction di\'ision to 

 beginners, although clear enough in themselves, often fail in 

 their purpose because the}^ do not take account of the fact that 

 reduction is indissolubly bound up with fertilization. To give 

 a clear conception of the significance of reduction it is necessary 

 to present the whole life cycle. In m.any respects the fern is 

 better suited than any other type for the representation of such 

 a cytological cycle. The alternation of generations is obvious; 

 the haploid as well as the diploid condition is evident; the an- 

 tithetic processes of fertilization and reduction occur at opposite 

 points of the life cycle and can thus be presented far m.ore clearly 

 than when reduction appears to be merely the "maturation of 

 the germ cells." 



The diagrams here presented are based on a hypothetical 

 fern with four chromosomes in the sporophyte. The cytolog}' 

 is that of Ascaris* very little schematized. Each of the chro- 

 mosomes of which two are represented as short and tw^o long, is 

 marked with a characteristic figure so that its pennutations 

 may be followed through the cycle. 



The best stage with which to begin is the diploid mitosis of the 

 sporophyte, which conforms to the familiar type of somatic karyo- 

 kinesis generally described. Omitting the resting nucleus the first 

 stage in division is the formation from the chromatin network 

 of a long, continuous spirem which winds in and out more or less, 

 filling the whole nuclear cavity (Fig. 1). Soon each granule of 

 this spirem divides and it becomes double longitudinally (Fig. 2). 

 After considerable contraction during which the chromatin 

 granules are drawn closely together, the spirem breaks into 

 four pieces, the chromosomes (Fig. 3). These are oriented on 

 the spindle and divided longitudinally along the line of the early 

 split (Fig. 4), one half going to each pole and entering into the 

 corresijonding daughter nucleus (Fig. 5), so that the progeny 

 of every chromosome is equally divided between the daughter 

 nuclei. As all of the cells throughout the organism are produced 

 in this manner each is exactly like every other in chromatin content 

 and, on the hypothesis that the chromosomes bear the hereditary 

 characters, in heritage as well. That this is actually the case 

 in the heritage as well as in the chromosomes may be demon- 

 strated by the familiar facts of vegetative propagation by which 



*See Griggs, R. F., A Rcilueing Division in Ascaris, Ohio i\at., 6: 519- 

 527. 1906. Wilson, E. B., The Cell, 2d Ed., pp. G5-72, 183. 236-242. N. Y. 

 1906. 



