564 A. C. WALTON 



Bpveri ('88, '92, '99), Hallez ('85), Meyer ('95), zur 

 Strassen ('96, '98), Ziegler ('95), and Zoja ('96) all have shown 

 that in A, megalocephala the first soma cell divides before the 

 stem cell and, with the exception of Zoja ('96), all state that the 

 phenomenon of 'diminution,' or 'chromatin reduction,' takes 

 place in the first division of the soma cell. With the exception 

 of Boveri and zur Strassen, these authors all state that only 

 four soma cells are produced in succession by the stem cell and 

 that each of the four undergoes a 'diminution' process at its 

 first division. Boveri, on the contrary, believes that there are 

 five such cells exhibiting 'diminution' divisions, and that it is 

 the sixth, not the fifth, division which results in pure propaga- 

 tion cells. Zur Strassen holds that there are six such 'diminution' 

 divisions in cells produced in succession by the stem cell. In A. 

 canis the evidence is not complete enough to warrant a positive 

 statement as to the exact number of ' diminution ' divisions 

 but embryos of about the forty-cell to fifty-cell stage show a 

 'diminution' spindle in the cell adjoining the stem cell, and that 

 is an argument for the view that this had been separated from 

 the stem cell at the previous (fifth) generation, and that it was 

 dividing to form a sixth generation of cells. 



Zoja ('96) states that the 'diminution' process in the case of 

 the first soma cell is often delayed until its second division, 

 namely, that of the daughter cells of the first soma cell. Thus 

 two ' diminution ' spindles are often seen at the same time as the 

 embryo goes from the four-cell to the six-cell stage. As the di- 

 vision resulting in the formation of the second soma cell does not 

 take place until somewhat later, there is no stage showing three 

 'diminution' divisions at one time. In A. canis this delay, 

 which is an occasional feature in A. megalocephala, is as frequent 

 as the earlier 'diminution.' 



'Diminution' consists, as was first shown by Boveri, in the 

 breaking up of the chromosomes and the extrusion into the 

 cytoplasm of portions of the chromatin occupying the ends of 

 each chromosome thread. The middle portion of each prophase 

 chromosome breaks into a number of small granules, which be- 

 come arranged in a metaphase plate (figs. 6-9 to 76), while the 



