274 



CELL-DIVISIOX AXD DEVELOPMENT 



ble changes of substance. The cause of the definite succession of 

 equal and unequal divisions is here wholly unexplained. 



Such cases prove that Balfour's law is only a partial explanation, 

 and is probably the expression of a more deeply lying cause, and 

 there is reason to believe that this cause lies outside the immediate 

 mechanism of mitosis. Conklin ('94) has called attention to the 



m- 



Fig. 125. — Embryos of the earthworm Allolobophoi-a fxtida, showing teloblasts or apical cells. 

 ./. Gastrula from the ventral side. B. The same from the riglit side; w, the terminal telo- 

 blasts ox primary 7nesoblasfs, which bud forth the mesoblast-bands, cell by cell; /, lateral teloblasts, 

 comprising a neuroblast, tib, from which the ventral nerve-cord arises, and two nep/iroblasts, n, of 

 somewhat doubtful nature but probably concerned in the formation of the nephridia. C. Lateral 

 group of teloblasts, more enlarged, the neuroblast, »/;, in division ; n, the nephroblasts. D. The 

 jjrimary rnesoblasts enlarged; one in division. 



