Feb., 1918] Sexual Dimorphism 105 



before the appearance of heterosporous sporophytes and that 

 it shows complete sexual segregation from the Heterosporous 

 Pteridophytes on up, while as stated, comparatively few 

 sporophytes have attained complete individual dimorphism. 



The gametophytes of Homosporous Pteridophytes exhibit 

 both hermaphrodite and unisexual species, and in the case of 

 certain unisexual ferns and horsetails, various experimenters 

 have shown that the sex can be controlled or reversed after it 

 is determined. The reversal can be brought about in either 

 direction, but generally a much larger per cent, of females 

 can be induced to produce male reproductive organs than 

 males can be changed to females. This is probably due to 

 the more specialized nature of the male soma, especially in 

 relation to nutrition and vegetative growth. It is well to call 

 attention here again to the fact that when the higher hetero- 

 sporous plants developed definitely unisexual, gametophyte 

 individuals, the segregation of the sexes was not accomplished 

 in or by the reduction division, as one might have expected, but 

 entirely independently of that process. All the four spores of a 

 reduction division have the same sexual tendencies and all give 

 rise either to male or to female gametophytes. Here the 

 development of males and females is associated with the size 

 of the spores. It is also interesting to note that in animals 

 the spermatoc^^tes through reduction give rise to four spermatids 

 which develop into spermatozoa without any reference as to 

 whether they contain an allosome or not. If the allosome is 

 what determines sex, how is it that the two spermatids without 

 allosomes develop maleness in exactly the same extreme way 

 as the two which contain the allosome? Certainly the peculiar 

 morphology of the spermatozoon must be considered maleness, 

 if there is such a thing. The same reasoning would apply to 

 the egg cell. In the case of the honey bee, all the oocytes 

 develop eggs, which we certainly must admit is a female expres- 

 sion; yet these same eggs without fertilization change their 

 sexual expression and develop into male individuals. In 

 animals we cannot test out the hereditary qualities of the 

 spermatozoa independently as we can the microspores of plants. 

 If we could we would probably obtain results analogous to those 

 coming from the androspores of certain species of Oedogonium, 

 which give rise to dwarf males. In the higher plants than 

 reduction of chromosomes has nothing to do with the maleness 



