52 



BIOLOGIC BASIS OF SEX 



Keller and Tandler (1916), Lillie (1917), 

 and the researches stimulated by their ob- 

 servations on cattle twins. The freemartin 

 in cattle develops in the same uterus with its 

 twin male. The blood circulations anasto- 

 mose so that blood and the products it con- 

 tains are common to both fetuses during de- 

 velopment. The development of the female 

 twin is intersexual, presumably because of 

 substances contributed by the male twin to 

 the common blood during uterine growth. 

 The freemartin intersexuality may be graded 

 into perfectly functioning fertile females to 

 types with external female genitalia and 

 typically male sex cords except germ cells 

 are absent, vasa efferentia, and elements of 

 the vasa deferentia. The conditions are simi- 

 lar to those discussed for amphibia, fish, and 

 rabbits in which early sex development 

 passes through neutral stages during which 

 it may be directed toward one sex or the 

 other by the right environmental stimuli. 



Intersexes in swine have been interpreted 

 as owing to similar causes (Hughes, 1929; 

 Andersson, 1956) although the resulting 

 phenotypes may not be quite as extreme. 

 The resulting intersexes for both cattle and 

 swine presumably are not caused by chromo- 

 somal misbehavior but to the right environ- 

 mental stimuli operating on suitable gene 

 backgrounds. The observations of Johnston, 

 Zeller and Cantwell (1958) on 25 intersexual 

 pigs all from one breeding group of York- 

 shires suggest significant inheritance effects. 

 The intersexes were of two types, "male 

 pseudohermaphrodites" and "true hermaph- 

 rodites," but there was some intergrading of 

 their phenotypes suggesting that they may 

 be the products of like causes. Common or- 

 gans between the two groups included uteri, 

 vulvae, vaginae, testes, epididymis, and 

 penis or enlarged clitori. The "true hermaph- 

 rodites" were separated on the basis of no 

 prostates, bulbo-urethral glands, or seminal 

 vesicles as well as having testes or ovotestes 

 with ovaries. A similar case was described 

 by Hammond (1912) but, as in one of the 

 above cases, the supposed ovaries when sec- 

 tioned seemed to be lymphatic tissue. Favor- 

 able nerve tissue^ from 6 of the Yorkshire 

 pigs was examined foi- nuclear chromatin. 

 The cases were found chromatin positive. 

 Phenotypically these cases also have paral- 

 l(>ls in mice and man. 



E. .SEX-iN man: chromosomal basis 



A surprise even to its discoverers, Tjio 

 and Levan (1956), came with the observa- 

 tion that the somatic number of chromo- 

 somes in cultures of human tissue was 46 

 rather than the previously supposed 48. 

 Search for the true number has been going 

 on for more than half a century. In early 

 investigations the numbers reported varied 

 widely. Difficulties of proper fixation and 

 spreading of the chromosomes of human 

 cells accounted for most of this variation 

 and the numerous erroneous interpretations. 

 Among the observations that of de Wini- 

 warter (1912) was of particular interest in 

 showing the chromosome number as 46 

 autosomes plus one sex chromosome with 

 the Y being absent. This number was also 

 found later by de Winiwarter and Oguma 

 (1926). Observations by Painter (1921, 

 1923) showed 46 chromosomes plus an X 

 and a Y, a total of 48. This number was 

 subsequently reported by a series of able 

 investigators, Evans and Swezy (1929), 

 Minouchi and Ohta (1934), Shiwago and 

 Andres (1932), Andres and Navashin 

 (1936), Roller (1937), Hsu (1952), Mitt- 

 woch (1952), and Darlington and Haque 

 (1955). As Tjio and Levan indicated, the 

 acceptance of 48 as the correct number, 

 with X and Y as the sex chromosome 

 arrangement, was so general that when 

 Drs. Eva Hanson-Melander and S. Kul- 

 lander had earlier found 46 chromosomes 

 in the liver cells of the material they 

 were studying they temporarily gave up the 

 study. In the few years since 1956, the ac- 

 ceptance of 46 chromosomes as the normal 

 complement of man has become nearly 

 universal. There are 22 paired autosomes 

 plus the X and Y sex chromosomes. 



The reasons which have warranted this 

 change of viewpoint are no doubt many, 

 but three improvements in technique are 

 certainly significant. The first came as a 

 consequence of simplifying the culture of 

 human somatic cells. The second followed 

 Hsu's (1952) recognition that pretreatment 

 of these cells before fixation with hypotonic 

 solutions tended to better spreads of the 

 chromosomes on the division plates when 

 subsefiuently stained by the squash tech- 

 niciuo. Pretreatment of the cultures with 



