OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 567 



any evidences of gastrulation or even of the ingrowth of the 

 propagation cells into the segmentation cavity. 



These observations, while scanty, go to prove that Ascaris 

 canis agrees with Ascaris megalocephala (Boveri, '99), in that 

 there are five stem cells which give rise to a soma cell and a 

 stem cell, the sixth stem cell giving rise only to pure propagation 

 cells. Each of these five soma cells, or its immediate daughter 

 cells, undergoes the process of 'chromatin diminution'. There- 

 fore, after the division of the soma cell derived horn, the stem 

 cell of the fifth generation, there is no further evidence of any 

 'diminution' processes of any kind, the soma cells multiplying 

 by ordinary mitoses as the propagation cells also do. Yet the 

 soma cells are distinguishable from the propagation cells by 

 their smaller and more numerous chromosomes. 



V. DISCUSSION 



The writer (Walton, '16 a) has shown that in the sperma- 

 togonia of Ascaris canis Werner there are thirty tetrad chromo- 

 somes — twenty-four autosomes and six idiosomes — as the diploid 

 number. By a process of pseudo-reduction in the prophase of 

 the first spermatocytes, this number is reduced to eighteen— 

 twelve di-tetrad autosomes and six tetrad heterochromosomes. 

 Both spermatocytic divisions are longitudinal. By the first 

 spermatocytic division, two types of second spermatocytes are 

 formed, one type having twelve tetrad autosomes and six tetrad 

 idiosomes, while the other has only the tetrad autosomes (twelve) . 

 These spermatocytes, by division, form spermatids, likewise of 

 two types, having either eighteen or twelve dyad chromosomes. 

 By direct metamorphosis the spermatids thus give rise to two 

 types of sperms. Further work ('16 c) has shown that each 

 sperm is provided with a refractive body of nutritive material, 

 upon which it subsists until union with an egg is accomplished. 

 The sperms also have cytoplasmic granules (mitochondria), which 

 are not 'plasma bearers of heredity.' The spermatozoa mature 

 rapidly at regular intervals, and copulation occurs immediately 

 if a female is present; if not, the spermatozoa are ejaculated 

 into the surrounding media. 



