REPRODUCTION 379 



gamete with an even number of chromosomes unites with an egg 

 cell, the result is a female, while if an odd-numbered one fertilizes 

 the egg, the result will be odd-numbered and a male. In Protentor 

 (a bug), the somatic number of chromosomes is thought to be 28 

 for the female and 27 for the male. There will thus be produced 

 egg cells with 14 chromosomes each and sperms with 14 and 13 

 chromosomes. If a sperm with 14 chromosomes unites with an 

 egg, the result will be a female with 28 chromosomes, while if a 

 sperm of 13 chromosomes fertilizes the egg, the zygote will de- 

 velop into a male. 



In plants, sex is not so distinct as in animals. Most plants 

 produce bisexual flowers with both kinds of gametes. Only a 

 comparatively small number of plants are dioecious, so that it 

 would seem difficult to connect sex with chromosome numbers as 

 in animals, but Allen (1917-1919) has found in the gametophytes of 

 Sphserocarpus, one of the liverworts, that the thalli which give rise 

 to archegonia (producing egg cells) have eight chromosomes, one 

 of which is a large X-chromosome; while the thalli which produce 

 antheridia and sperms have, instead of the large X-chromosome, 

 a small Y-chromosome. The sporophyte contains, of course, 

 both the X- and the Y-chromosomes, which are separated when 

 the asexual spores are formed; those spores which obtain the X- 

 chromosome develop into female thalli while those which contain 

 the Y develop into male thalli. Miss Blackburn (1923) reports 

 what seems to be chromosome sex differentiation in Lychnis and 

 Populus, the female having an XX complex and the male an XY. 

 There are thus produced two kinds of male gametes. In the com- 

 paratively few (about a dozen) genera where sex in plants is 

 demonstrably connected with the germinal constitution, the ho- 

 mozygous condition is generally in the female and the heterozygous 



in the male. 



While the chromosomes are connected with sex in the forms 

 cited, it remains to be proved that the chromosomes determine 

 sex. Are they the cause or merely an accompaniment? There is 

 plenty of evidence to show that sex is determined not only by 

 the germinal constitution but also by the environment. In the 

 animal kingdom, the work of Riddle (1914-1917) is classic in this 

 field of study. He has shown that in pigeons which lay two eggs to 

 a clutch, the egg which is laid first develops into a male bird and 

 the second egg into a female. The first egg has less stored food, 



