144 The Ohio Naturalist. [Vol. X 11 1, No. 7, 



mosomes is larger as is the case in most organisms and each of the 

 chromosomes is oriented by chance independently of the rest 

 as is presumably the case it is obvious that the number of coin- 

 binations i. e. the nirmber of kinds of reduced cells increases as the 

 square of the number of chromosomes. 



Omitting the variations, however, and following one of the 

 nonsexual spores, say that with chromosomes marked with 

 circles and dots, we find that it produces on gennination the 

 familiar heart-shaped gametophyte (prothallus) of the fern. 

 The mitoses occurring in the growth of this plant (Figs. 14 and IS), 

 are exactly similar to those of the sporoph}'te except that they 

 have only the reduced number of chromosomes found in the 

 spore from which it grew, i. e. they are haploid instead of diploid. 

 When mature the gametophyte produces archegonia bearing 

 eggs, and antheridia bearing sperms. In the development and 

 maturation of these gametes there is, of course, no reduction 

 division. 



Fertilization may occur between an egg and a sperm from the 

 same plant or the sperm may come from a different gametophyte. 

 The latter alternative is figured in the diagram and it is further 

 assumed that the sperm came from a gametophyte derived from 

 a spore bearing the chromosomes marked with crosses (Figs. 

 20 and 21). When the sperm fuses with the egg their nuclei 

 may be in a resting condition or they may be resolved into their 

 respective chromosomes (Figs. 19-22). and proceed at once into 

 the first mitosis of the succeeding embryo and the cvcle is com- 

 plete. (Figs. 23-25). 



The significance of the conventions adopted in marking the 

 chromosomes thus becomes apparent. Those marked with dots 

 and circles came from the egg parent and those niarked with 

 crosses from the sperm parent. In view of this, the fact com- 

 mented upon above that each chromosome pairs with its ap- 

 propriate mate in synapsis, takes on a new significance, for each 

 of the tetrad or reduction chromosomes is seen to consist of a 

 doubled chromosome of maternal origin paired with the cor- 

 responding one of paternal origin. It is also evident that 

 while the nuclei fvise in fertilization, the chromosomes do not 

 show an}' sexual affinit}' for each other and live together, so to 

 speak, in the nuclei of the diploid generation as independent units, 

 until in the first half of the reduction division the corresponding 

 pairs of maternal and paternal chromosomes appear to develop an 

 attraction for one another and finally unite as synaptic mates to 

 form the reduction chromosomes, so completing the union of 

 sexual elements begun at the time of fertilization. 



It is obvious, moreover, that if by chance one of the chromo- 

 somes had been oriented differently in the reduction division, as 

 indicated by the alternative Figure 10a, none of the spores result- 



