SEX 



359 



If, on the other hand, an egg is fertilized by a sperm with a heterochromo- 

 sorne, the resulting individual receives two, one from each gamete, and 

 this individual develops into a female. 



In Ancyracanthus (Mulsow 1912) (Fig. 138) the male has a single 

 heterochromosome which, since it has no homologue with which to pair, 

 passes to half the sperms, while in the female there are two such elements,' 

 every egg receiving one. The two types of union result in individuals 

 of the two sexes, as in Ascaris. In Ancyracanthus Mulsow states that 

 the five and six chromosomes can actually be counted in the living 

 spermatozoa. 



FIG. 138. The behavior of the sex-chromosomes in Ascaris (Boveri), Ancyracanthus 

 (Mulsow, 1912), Lygceus (Wilson, 1905), and Prionidus (Payne, 1909). 



In Lygceus (Wilson 1905) (Figs. 138; 139, A) there are in the male 

 two heterochromosomes, one small and one large (an " XY" pair); in the 

 female there are two large ones ("XX"). Half the sperms receive the 

 X and half the F, and every egg has an X. Fertilization by an X sperm 

 results in a female (XX), and by a F sperm in a male (XY). 



In Prionidus (Payne 1909 1 ) (Figs. 138; 139, B) the male has three 

 small heterochromosomes and also a much larger one. At the time of 

 reduction the three small ones behave as a unit and pair with the large 

 one: half of the sperms therefore carry the former and half the latter. 

 In the female there are six small heterochromosomes, and the eggs are all 

 alike in having three each. Fertilization now results in females with 

 six small elements and males with three small and one large. 



1 In this paper Payne gives diagrams of several other types of heterochromosomes. 



