578 A. C. WALTON 



of the soma cells arising by the first five divisions of the stem 

 cell, and except for the soma cell of the first generation, the proc- 

 ess always occurs in their first subsequent division. The 

 amount of chromatin thus lost is about equal to the amount 

 retained, and the extruded masses of material can be seen for 

 some time, since their disintegration in the cytoplasm is slow. 

 These masses are commonly located next to the cell wall which 

 was formed in the plane of the equatorial plate of the 'diminu- 

 tion' spindle. 



The sixth division of the stem cell gives two purely propaga- 

 tion cells, whereas each of the five previous divisions had given 

 rise to a stem cell and a soma cell. From the two propagation 

 cells come all the sexual cells of the embryo and the adult. As no 

 ' diminution' takes place in the propagation cells at any division, 

 and as all the soma cells have arisen from cells which have un- 

 dergone this process, it is easy to recognize the primitive germ 

 cells from the body cells, not only by their greater amount of 

 chromatin, but also by the fact that they still have only thirty 

 or thirty-six (sexual difference) dyad chromosomes, whereas the 

 soma cells all have sixty or seventy-two small monad chromo- 

 somes. 



VI. SUMMARY 



The following points have been brought out in this study of 

 the oogenesis of Ascaris canis Werner. 



1. The diploid number of chromosomes is thirty-six; the hap- 

 loid eighteen. 



2. There is a heterochromosome group of the '2X' type, con- 

 sisting of twelve tetrad chromosomes. 



3. Each of the chromosomes of the oogonia— diploid number — 

 is potentially a dyad and becomes a tetrad by a longitudinal 

 division during the early first oocytic prophase. 



4. By a process of pseudo-reduction through parasyndesis 

 these thirty-six (diploid number) tetrad chromosomes are re- 

 duced to eighteen di-tetrads (haploid number) in the late pro- 

 phase of the first oocyte. 



5. Of the two oocytic divisions, the first is equational, the 

 second reductional, in the Weismannian sense. 



