246 



The Journal of Heredity 



to develop parthenogenetically (with- 

 out fertilization), but produce half males 

 and half females if fertilized, the result 

 beinji; due to two kinds of si)ermatozoa, 

 male-i:)roducinj^ and female-producing. 

 In still other cases, notably the hymen- 

 optera (bees, wasps, ants, etc.), males 

 are produced when the eggs are not 

 fertilized and females when the eggs 

 are fertilized. All of these apparently 

 divergent phemonena are consistent 

 with the idea that sex is determined in 

 the germ-cell and that the sex-deter- 

 mining factor is in some way intimately 

 associated with the presence of a pecu- 

 liar chromosome (the X-chromoscme). 

 or group of chromosomes, in the nucleus 

 of the germ-cell. This mechanism gives 

 a sex-bias to the individual, a bias in 

 some cases so strong that no known fac- 

 tors can interfere with the fulfillment of 

 the sex development that was originally 

 determined. In other cases, however, 

 sex may be determined at the time the 

 egg is fertilized, but may require a 

 definite favorable environment to bring 

 it to complete development or differen- 

 tiation. Finalh', in some cases the 

 individual whose sex has been deter- 

 mined as a male, say, in the fertiliza- 

 ti(Mi of the egg, may have its sex devel- 

 (jjjment so altered by outside pressure, 

 as to be almost indistinguishable from a 

 female at maturity.^ 



In mammals there is much evidence 

 that sex is definitely determined at the 

 monent the egg cell is fertilized by the 

 spermatozoon. There appear to be two 

 kinds of sjjermatozoa and but one kind 

 of egg, and the sex of the individual 

 depends on whether a male-]:»roducing or 

 a female-producing spermatf)Zf:(Jn fer- 

 tilizes a particular egg. 



If, then, sex in mammals is deter- 

 mined in the undeveloped egg, two or 

 more individuals derived from a single 

 egg would naturally be expected to have 

 the same sex. This is just the ]joint 

 upon which a study of twinning bears, 

 for dujjlicate twins are, in efTect, ex])eri- 

 ments demonstrating the correctness 

 of the theory of sex-determination which 

 is now held. 



If a fertilized egg could be artificialh' 

 divided into two or more parts, and the 

 individuals developing from these sepa- 

 rate parts were always of the same 

 sex, the current theory of sex-deter- 

 mination would be regarded as proved 

 by exi)eriment. The most refined tech- 

 nique is not yet adequate to carry out so 

 crucial an experiment, but nature pro- 

 vides an equivalent one. It has been 

 shown conclusively for two species of 

 arn^adillo, and by analogy for man, that 

 an egg, divided at an early ])eriod, pro- 

 duces individuals all of the same sex. 

 In hundreds of sets of quadruplets of the 

 Texas armadillo there has occurred no 

 exception to this rule, in spite of the 

 fact that in some cases there are marked 

 differences in size of the offspring, due 

 to vmequal environment factors. In 

 one case two fetuses of a set are nearly 

 twice the size of the remaining two, yet 

 the sex of all is the same, showing that 

 the sex determined at the moment 

 of fertilization is incapable of altera- 

 tion through any ordinary change of 

 amount of nutrition and the like. 



This generalization holds good in 

 other animals than mamimals. Lito- 

 mastix, a parasitic wasp, lays its eggs 

 in the body of a caterpillar, and a single 

 egg divides very early into a large num- 

 ber of jjarts — as many as a thousand — 

 each of which produces an adult insect. 

 But these adults are always of the same 

 sex. 



If then in two groups as far apart as 

 mammals and insects the fact of sex 

 determination at the time of fertiliza- 

 tion is ]M-oved by twinning, it seems prob- 

 al)le that this is a very general i)rinciple. 

 ])()ssil)ly almost universal. 



si:x-diffi-:rkntiation in mammals 



Although sex is thus determined in 

 mammals, the dififerentiation of sex- 

 characters depends on a secondary 

 mechanism that is believed to be asso- 

 ciated with an internal secretion of the 

 reproductive organs. It has been long 

 known that castrati(Mi of young mam- 

 mals i)revents the develoi^nent of adult 

 sexual characters, and the individual re- 



' For an account of llie way lliis can he done experimentally in pigeons see "Success in Con- 

 trollinj^ Sc>x" (an account of the work of ( )scar Rirlrlle, and the late C. ( ). Whitman), in the JouRN.xu 

 OF Hkkkditv, vii, ])p. l.SS-164, April, l'M6, 



