is so frequently the case in P. equorum, it is attached to 

 the end of an autosome and only occasionally is distinct 

 enough for positive identification. In most cases the 

 hetero.hrc mosome undergoes "pre-reduction" as do the 

 autosomes, but in some cases it shows "postreduction" 

 although the autosomes seem to show the differential 

 division as being the first. This may point to the primitive 

 condition being actually one of "postreduction" (Edwards, 

 1910; Wilson, 1925, p. 757). In many instances the 

 heterochromosome (or heterochromosome complex) either 

 precedes the others or lags behind during one or both of 

 the meiotic divisions and in some cases forms a separate 

 chromatin nucleolus during the interphase stage. Where 

 both "X" and "Y" are present, they are separated most 

 frequently at the first division, each undergoing equational 

 splitting at the second. In the early Spermatocyte I 

 growth period nuclei, the "XY" group is differentiated 

 from a single chromatin nucleolus and the "XY" pair 

 assumes a "tetrad" form, usually asymmetrical because 

 of the small bulk of the "Y" element. Even when the 

 "X" is multiple it differentiates from a single nuclear 

 body (Walton, 1916, 1924), just as it does when "X" = 1. 

 The nematodes therefore afford a wide variation of 

 heterochromosome types, varying from a single "X" and 

 no "Y" in Heterakis dispar and Cystidicola farionis 

 (Anct/racanthus cystidicola) to forms like Toxoeara vulpis 

 (Belasearis triquetra) and Heterakis spumosa (Gangulet- 

 erakis spumosa) with an "X"-complex of two, Ascaris 

 lumbricoides with one of five, Toxoeara canis (Toxascaris 

 car-is) with one of six, and Parascaris equorum with one 

 of eight to nine, and no "Y", and thence to forms such 

 as Contracaecum incurvum with an "X"-complex of eight 

 and a single "Y". Peculiarly, no established case of an 



"XY" pair has been definitely recognized. The "X" and 

 "Y"-chromatin may form a single body or a single unit 

 during meiosis, just as frequently the autosomes may 

 conceal their complexity temporarily in single bodies 

 under the same circumstances (P. equorum).* 



The following chart gives the majority of the examples 

 of the species which have furnished material for the 

 study of nematode gametogenesis. In each case the 

 haploid and diploid chromosome numbers are indicated, 

 the somatic number is given, and the form of the 

 chromosomes at each stage (di-tetrad, tetrad, dyad, 

 monad) is noted. Wherever the germinal chromosomes 

 are plurivalent, their unit value in terms of the somatic 

 chromosomes is pointed out. The nature and number of 

 the heterochromosomes is indicated for each species as 

 far as it is known. The presence or absence of the 

 "diminution" process is also stated for such species as 

 have been examined for that phenomenon. 



•Jeffrey and Haertl (1938) have recently questioned the whole 

 subject of "sex chromosomes" in nematodes in their study on 

 Asraris lumbricoides, Toxoeara cati, T. canis, and "Ascaris" spt 

 from a seal. They fail to find any evidence of a consistant dif- 

 ferential distribution of what might be called "X-chromatin". and 

 show that, while certain chromosomes lag behind in each meiotic 

 division, these are not necessarily distributed as "X" and "O". 

 or "X" and "Y" materials must be. Analogous behavior of 

 chromosomes is known to occur in various hybrid forms of plants 

 and animals, and is evidence of their mixed ancestry. The authors 

 argue that these nematodes, and probably all similar forms, are 

 likewise hybrids because of the evidence presented by the behavior 

 of their chromosomes, particularly during the process of meiosis. 

 If the nematodes are hybrids, then the uneven distribution of the 

 chromosomes during the maturation divisions is to be expected, 

 and is a clue to their hybrid ancestry, not a proof of the presence 

 of "sex chromosomes" as has been the usually accepted interpretation. 



List of Abbreviations 



A, Cell formed by the division of the 1st generation Soma 



cell. 



B, Cell formed by the division of the 1st generation Soma 



cell. 

 c, Centrosome. 

 eh, Chromosomes. 

 EM, Cell formed by the division of the 1st generation 



Stem cell. 

 g. Germ cells. 

 </o, Golgi body. 

 rr, Golgi ring. 

 h, Heterochromosome. 

 ic, Intranuclear centrosome. 

 /, Cytoplasmic lobe. 

 m. Cell membrane. 

 mi, Mitochondria. 

 vcl, Nucleus. 

 P /, 1st. polar body. 

 P //, 2nd. polar body. 

 PI, 1st. generation Stem cell. 

 P2, 2nd. generation Stem cell. 

 pg, Pre-acrosomal granule. 

 Pi', Perivitelline space. 

 rb, Refringent body, or Acrosome. 

 re. Refringent corpuscle. 

 rg, Refringent globule. 

 SI, 1st. generation Soma cell. 

 s, Spermatozoon. 

 sh, Egg shell. 

 <;?■, iSperm remnant. 

 v, Fertilization membrane. 

 vm, Vacuolated mitochondria. 

 X, X-chromosome. 

 Y, Y-chromosome. 



Male sign, Male pronucleus, or of male origin. 

 Female sign, Female pronucleus, or of female origin. 



ill 



