OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 569 



The cleavage divisions follow in rapid order, exhibiting the 

 characteristic nematode 'diminution' of chromatic material in 

 the first division, or the following one, of each soma cell after it 

 has been cut off from the stem cell. The three-cell and four-cell 

 embryos show the curious 'T' shape first seen in the embryos of 

 A. megalocephala. In the four-cell stage, a rotation of the stem 

 cell occurs, giving a lozenge-shaped embryo. In the propaga- 

 tion (stem) cells, the chromosomes retain their dyad character, 

 being either thirty or thirty-six in number. In the soma cells, 

 after 'diminution', the chromatin has the form of monad chro- 

 mosomes — sixty or seventy-two in number depending upon the 

 sex of the embryo. The propagation cell is one of the cells in 

 the upright of the 'T' embryo. Often the division of the first 

 soma ceil is unaccompanied by the process of 'diminution,' this 

 phenomenon being delayed until the division of the two daughter 

 cells of the first soma cell, the three-cell embryo thus showing 

 two, instead of one, 'diminution' spindles at the same time. 

 Since the second stem cell divides before the second soma cell, 

 four-cell embryos are occasionally found which show the two 

 'diminution' spindles and also the normal spindle figure of the 

 dividing stem cell. 



The present paper on the oogenesis of A. canis, together with 

 that on the spermatogenesis of the same species (Walton, '16 a), 

 gives adequate grounds for stating that the dog ascaris studied 

 by Marcus ('06) was not Ascaris canis Werner, although Marcus 

 so regarded it, but was most probably (Walton, '16 b) Ascaris 

 triquetra Schrank — also an occasional intestinal parasite in dogs; 

 for A. triquetra differs both in gross anatomy and in cytological 

 details from the conditions found by the writer in A. canis, but 

 agrees cytologically with the conditions set forth by Marcus 

 ('06) for A. canis. 



The results of the present study have led to several questions 

 of interest. The chromosome cycle of A. canis is one of these. 

 Study of the spermatogenesis had shown that A. canis Werner 

 is one of the few Nematodes which posses a peculiar eight-parted 

 chromosome at the beginning of the first maturation division. 

 The two spermatocytic divisions result in a chromosome, which 



