358 INTRODUCTION TO CYTOLOGY 



Sex-Chromosomes. The theory of the automatic determination of 

 sex and its relative, if not absolute, fixity has had one of its strongest 

 supports in the results of certain researches on the spermatogenesis of 

 animals. In 1891 Henking noticed in certain insects that half of the 

 spermatozoa contain an extra body, which he thought might be a 

 nucleolus. It was subsequently shown by Paulmier (1899), Montgomery 

 (1901), and de Sine'ty (1901) that this body is not a nucleolus but an 

 extra or "accessory" chromosome. Henking's misinterpretation had 

 apparently been due to the fact that the accessory chromosome often 

 does not transform into a portion of the reticulum along with the 

 other chromosomes ("autosomes"), but remains condensed and closely 

 resembles a nucleolus. Half of the spermatozoa in these animals there- 

 fore have one more chromosome than the others: hence the male is 

 said to be "heterogametic," or "digametic." It was at once suggested 

 by McClung (1902) that the accessory chromosome in some way 

 determines sex that eggs fertilized by one kind of spermatozoon 

 develop into females, while those fertilized by the other kind become 

 males. This represents the first attempt to connect a given character 

 with a particular chromosome. An extensive series of researches was 

 now undertaken by Wilson, Miss Stevens, McClung, and a number of 

 other cytologists, who discovered among insects many striking instances 

 of the phenomenon. Accessory chromosomes (also referred to as sex- 

 chromosomes, heterochromosomes, idiochromosomes, x-chromosomes, 

 x-elements, and supernumerary chromosomes) of a number of different 

 types were found, not only among insects, where they are best displayed, 

 but also in certain echinoderms, nematodes, mollusks, and vertebrates, 

 including birds and man. A number of representative cases will now 

 be described. 



Male Heterogametic. In the threadworm, Ascaris (Boveri) (Fig. 138) ! 

 there is in each body cell and primary spermatocyte of the male a single 

 heterochromosome, which seems to be attached to, or to constitute a 

 portion of, one of the four autosomes. At the time of reduction this 

 passes undivided to one daughter cell at the first division and divides at 

 the second, so that half of the sperms only receive it. In the female there 

 are two such heterochromosomes, every egg receiving one. If, now, an 

 egg is fertilized by a sperm without a heterochromosome the resulting 

 individual has only one (that from the egg) and develops into a male. 



1 For the sake of brevity and clearness these diagrams are drawn as if only one 

 maturation mitosis occurred in spermatogenesis and oogenesis. It will be understood 

 that there are two divisions, resulting in four sperms instead of the two shown, and 

 in an egg and three polar bodies instead of the two eggs shown. The diagrams merely 

 indicate that two sorts of sperms and one kind of egg are produced, and how this 

 is brought about. In the cases of Lygceus and Prionidus the number of autosomes 

 shown (4) is not the actual number present. See the review of the subject of sex 

 chromosomes by Wilson (1911). 



