534 A. C. WALTON 



presence or absence of the six heterochromosome dyads. The 

 oogonia were found to have eighteen di-tetrad chromosomes as 

 the diploid number. The heterochromosomes were present in 

 the '2X' condition in the female. 



Further work (Walton, '16 c) showed that the refractive body of 

 the spermatozoon was purely nutritive in function, and that it 

 arose by the fusion of the refringent granules of the spermatids. 

 The mitochondria were shown to have had a nuclear origin and 

 to have served for a while as centers about which the refringent 

 granules of the yolk Avere built up. Later the mitochondria 

 were grouped around the centrosomes in a cap-like mass, passing 

 through the nuclear reduction divisions unaffected. They main- 

 tain this position in proximity to the centrosome when the sperm 

 enters the egg and then break up and become lost to view as the 

 spermatozoon disintegrates. Hence, they seem to have played 

 no part as bearers of hereditary characters. 



The writer (Walton, '16 b) found that A. triquetra Schrank 

 occurred very rarely in dogs, but had been several times reported 

 as common in bears. An examination of the gametogenesis of 

 this parasite showed a very close correspondence with the con- 

 ditions found by Marcus ('06) in A. canis. In view of the source 

 of most of Marcus's material, and of the correspondence of cyto- 

 logical conditions between the form studied by him and A. tri- 

 quetra, it seemed probable that the nematodes used by Marcus 

 were A. triquetra Schrank, rather than A. canis Werner. 



The work of the present writer has thus shown, not only by 

 morphological, but also by cytological evidence, that A. canis 

 and A. felis are different species. 



The literature on other nematodes, especially Ascaris megalo- 

 cephala, is so enormous that no attempt at even outlining its 

 results has been made. Wlierever pertinent, however, detailed 

 reference to the conditions in other nematodes is made at ap- 

 propriate points throughout this paper. In the study of the 

 plasmosome, reference has also been made to certain works on 

 the gametogenesis of insects. The work of Jorgensen ('10) has 

 had special interest because he finds in the gametogenesis of 

 certain Sycon sponges di-tetrad chromosomes which reduce to 



