528 A. C. A^'ALT()N 



the history of the de\'elopnieiit of the nuclear and cytoplasmic 

 elements of the egg, — especially in regard to the method of the 

 formation of the peculiar di-tetrad autosomes, and also in I'e- 

 gard to the identity of the heterochromosome complex repre- 

 sented in the male by a group of six tetrad chromosomes, — and 

 also to follow the process of fertilization and early stages of 

 segmentation. 



The material for this work was obtained exclusively from 

 females of A. canis found in the intestines of dogs, and care was 

 taken that only specimens of this species were employed. This 

 precaution is important, for there is at least one other s}:)ecies of 

 Ascaris (A. triquetra Schrank) found in the intestines of the dog 

 which may be mistaken for A. canis Werner. 



The writer here wishes to express his thanks and appreciation 

 to Dr. E. L. Mark, not only for his criticism and advice in the 

 carrying out of the investigations, I ut also for his assistance in 

 obtaining material and for his care in criticizing the final form of 

 this paper. Thanks are also due to Dr. S. I. Kornhauser, North- 

 western University, for personal notes based on his examination 

 of the material prepared by Marcus (/06) for the study of the 

 oogenesis and spermatogenesis of an Ascaris which he wrongly 

 designated as A. canis Werner, but which in fact was A. tri- 

 quetra Schrank (Walton, '16 b). The writer is also indebted 

 to Dr. Kornhauser for the use of slides loaned from the North- 

 western University collection. 



II. LITERATURE 



For many years Ascaris canis Werner has been an object of 

 study by the medical profession as a parasite in both domestic 

 animals and man, but as a subject for taxonomic, morphological, 

 and cytological investigation, it has been much neglected, only 

 a few, widely separated sporadic efforts in these directions hav- 

 ing been made. The identity of the nematode now known as 

 A. canis Werner was for many years a mooted question among 

 taxonomists. Nematode parasites superficially alike have been 

 found not only in the dog and the cat, but also in other members 

 of the families Canidae and Felidae. These were known by a 



