376 INTRODUCTION TO CYTOLOGY 



the other members of the complement are called autosomes. Typical 

 examples of sex-chromosomes in animals are given below. ^ 



In 1891 Henking noticed that half of the spermatids of Pyrrhocoris 

 apterus, an insect, contain a body which he thought might be a nucleolus. 

 It was subsequently shown- that this supposed nucleolus is an "accessory 

 chromosome" which remains compact while the other chromosomes 

 become diffused or reticulate in form. That this accessory chromosome 

 exerts a special influence in sexual differentiation was suggested by 

 McClung (1901, 1902) and proved by him and many other observers. 



When some of the sperms have one sex-chromosome (X) and the 

 others none, the animal is said to show the " XO type" of sex-chromosome 

 differentiation. A good example is Protenor helfragei, a bug'' (Fig. 212). 

 In the somatic nuclei and spermatogonia of the male there are six pairs 

 of autosomes and a single unpaired X-chromosome. The autosomes 

 undergo disjunction in / as usual and divide equationally in //, but the 

 X-chromosome divides in I and passes undivided to one pole in //. 

 Two sperms of the quartet receive one X each, while the other two 

 receive none. In the female somatic cells and the oocytes there are two 

 X-chromosomes besides the autosomes; every egg receives one X. 

 Fertilization by a spermatozoon with an X results in a female with two 

 Xs, whereas that by one with no X results in a male with only one. In a 

 number of insects, domesticated mammals, and lizards, the X-chromo- 

 some passes undivided to one pole in / and divides in //, the end result 

 being the same as in Protenor.* The plotted measurements of the 

 spermatozoa of Canis form a bimodal curve, the indicated size dimor- 

 phism probably being associated with the chromosomal difference.^ 



Two variants of the XO type are found in the nematode genus Ascaris. 

 In A. megalocephala bivalens (Fig. 212) there are in the spermatocyte 

 four autosomes and a single small X, which is usually attached to the 

 end of one of the autosomes. The X passes undivided to one pole in / 

 and divides in //; hence it is present in half of the spermatozoa. In 

 the oocyte there are two such X-chromosomes, which disjoin in / and 

 divide in II, every egg thus receiving one. Fertilizations of eggs by 

 sperms with and without an X result in females and males, respectively. 



1 For extensive reviews, see Crew (1927c), F. Schrader (1928), Wilson (1925), and 

 Witschi (1929). See M. Hartmann (19296, 1930) on Protista. 



2 McClung (1899), Paulmier (1899), de Sinety (1901), Montgomery (1901). 



3 Montgomery (1901), Wilson (1906), Morrill (1910). 



■* The behavior of sex-chromosomes in / and // is tabulated by Wilson (1925). A 

 tendency on the part of the X to lag may result in both modes of distribution in the 

 same species (Edwards, 1910). 



5 Malone (1918) on the dog. Other reports of XO type: Wodsedalek (1913, 1914, 

 1920, 1922) on horses, sheep, pigs, and cattle; Masui (1919a6) on horses and cattle; 

 Painter (1920, 19216) on lizards; GuHck (1911) and Mulsow (1912) on nematodes; 

 Brauer (1928) on Bruchus; and Mols (1928) on guinea pig. See also next footnote. 



