CELL-DIVISION 



lack centrosomes, as we have seen in the chapter on the 

 fertilization-process, other eggs, indeed most animal eggs, 

 possess them. The reason for this difference is here beside 

 the point. The difference exists and its existence warns 

 us to be wary in drawing too final and dogmatic conclusions 





a b c 



Fig. 34. — Cell-division, living cells of Triton (after Peremeschko). Note changes 



in the ectoplasm. 



with respect to these components. Moreover, were we to 

 compare the first cleavage-stage in other eggs with that in 

 sea-urchins', we should find no strict correlation of the 

 appearance of the cleavage-furrow with either the phase of 

 mitosis or the size of the centrosphere-aster complex. When 

 the egg of Ascaris^ for example, 

 divides, the chromosomes in 

 early telophase are still strongly 

 stained; no such pronounced 

 centrospheres as in sea-urchins' 

 eggs are present. Briefly said, 

 furrowing of the cytoplasmic 

 mass of eggs does not occur 

 always at precisely the same 

 stage of mitosis in all species 

 of them. Nor cioes it occur 

 uniformly In other animal cells. 

 (Note for example Fig. 34 and Fig. 35.) This is thus my 

 first point: We must be sure by exact observation as to what 

 happens in sea-urchins' eggs; but with this knowledge we 

 are not justified in drawing conclusions concerning division 



265 



■■■i& 



^ - m 



my 



a b 



Fig. 35. — Stages in cell-division, 

 spermatogenesis of Pyrrhocoris aper- 

 tus (after Gross). 



