274 



CELL-DIVISION AND 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 



C 



Fig. 125. Embryos of the earthworm Allolobophora fcetida, showing teloblasts or apical cells. 

 A. Gastrula from the ventral side. II. The same: from the right side; w, the terminal telo- 

 blasts or primary mesoblasts,\\\\\c\\ bud forth the mesoblast-bands, cell by cell; t, lateral teiobhisls, 

 comprising a neuroblast, nb, from which the ventral nerve-cord arises, and two nephr of/lasts, n, of 

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

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

 primary rnesoblasts enlarged; one in division. 



