Il8 CHARLES V. MORR1LL. 



Outside of the air-breathing arthropods, there are, as mentioned 

 before, two other groups in which idiochromosomes or similar 

 structures have been found in maturation and cleavage. Baltzer 

 ('08) has found that in two species of sea-urchins there is a par- 

 ticular hook-shaped chromosome which occurs in only a part of 

 the mature eggs. The eggs are thus of two types with respect 

 to this element. (It is replaced by a chromosome of the ordinary 

 sort in the eggs which lack it.) The sperm nuclei on the contrary 

 are all alike. It is not improbable, Baltzer concludes, that the 

 determination of sex depends upon this dissimilarity of egg nuclei, 

 and therefore lies with the female (i. e., with the egg), as in the 

 male and female (parthenogenetic) eggs of aphids and phyllox- 

 erans. The peculiar hook-shaped elements might thus be called 

 "idiochromosomes." Eggs which contain this element would 

 develop into females, those without, into males. In a very 

 recent paper Boveri and Gulick ('09) have described briefly the 

 chromosome-cycle in Heterakis, a nematode. Its cycle corres- 

 ponds exactly with that of Protenor as given by Wilson ('06). 

 The diploid number in the male (spermatogonia) is 9. During 

 spermatogenesis the odd chromosome goes undivided to one pole 

 of the spindle in the first spermatocyte division but divides in 

 the second. The spermatozoa are thus of two classes, with 5 

 and 4 chromosomes respectively. The diploid number in the 

 female was not determined with certainty but the haploid number 

 in the germinal vesicles and polar spindles was found to be 5. 

 The eggs are thus all alike and, it is assumed, will develop into 

 males or females according as they arc fertilized by 4-chromosome 

 or 5-chromosome spermatozoa. The chromosomes of the cleav- 

 age nuclei were not described. 



Since the results here described for coreid Hemiptera do not 

 give any further insight into the fundamental question of sex- 

 determination but only render the data more complete, it seems 

 needless to add a lengthy discussion on this point. In the recent 

 papers of Wilson, Bateson Castle, Boveri and Morgan, the cyto- 

 logical evidence relating to sex-determination has been thoroughly 

 analyzed. It may be pointed out, however, that apart from theo- 

 retical considerations this evidence has been questioned from tin- 

 standpoint of fact by several workers who have supported their 



