ASCARIS CANIS AND ASCARIS FELIS. 367 



In the specimens examined by the writer there were none of the 

 variations claimed by the writers last mentioned, and therefore 

 the present writer maintains upon a morphological basis that the 

 Ascaris of the dog is a species distinct from the Ascaris of the cat. 



SPERMATOGENESIS OF A. CANIS AND A. FELIS. 



In the present article only very characteristic stages in the 

 spermatogenesis will be considered. 



i. A. canis. 



The chromatin of the spermatocytes nearing the end of the 

 growth period appears in the form of two irregular masses situ- 

 ated peripherally in the nucleus. One mass is small and circular, 

 whereas the other is long and narrow, lying within the nuclear 

 membrane and extending along half of its circumference (Plate I., 

 Fig. i). There is also a plasmosome which loses its affinity for 

 stains and disappears shortly after the formation of the prophase 

 chromosomes. The two chromatin masses early assume a 

 vacuolated appearance and become distinct chromosomes. 

 From the larger chromatic mass are formed twelve large chromo- 

 somes; and from the smaller karyosome are formed six small 

 chromosomes, closely grouped. Thus we find early that the 

 haploid number of chromosomes, eighteen, clearly defines itself. 

 The group of twelve large chromosomes retains a peripheral 

 position during the prophase, while the smaller group migrates 

 early to the central part of the nucleus. 



The centrosome is of extranuclear origin as it sets up the 

 mitotic figure before the nuclear membrane disappears (Plate I., 

 Fig. 2). The spindle fibers seem to arise from the centrosome 

 and push against the nuclear membrane, which often shows 

 irregular indentations in the region of the centrosome. 



In the metaphase of the first spermatocyte division the chromo- 

 somes are arranged in the equatorial plate, with the group of 

 six chromosomes in the middle and the twelve larger chromo- 

 somes scattered more peripherally (Plate I., Fig. 3). From a 

 polar view these peripheral chromosomes all show a distinct 

 tetrad form. From a lateral view, careful study reveals that 

 the twelve large chromosomes also show a tetrad form (Plate I., 



