320 



EMBRYOLOGY 



same size (Fig. 151 5), one of which Whitman designates as 

 neuronepliroblast, and the other as mesoblast, in accord- 

 ance with their subsequent fate. The mesoblast soon divides 

 into two cells, which at first do not occupy bilaterally sym- 

 metrical positions, as would be expected of the primitive 

 cells of the mesoderm. One of them lies more behind, the 

 other more in front beneath the micromeres, the number of 

 which soon increases, first at the expense apparently of the 



Fig. 151.-/4 to C, cleavage stages of Chpsine, diagrammatic (after Whitmvk). 

 I. and II. indicate the direction of the first and second planes of division ; a, h, c, the 

 macromeres which become entoblasts ; k, the macromere which supplies the germ 

 bands ; a', h', c', k', micromeres which arise from the macromeres a, b, c, and k ; 

 m, mesoblast ; nn, neuronephroblast ; n, neuroblast; np, nephroblasts ; I, pole cells 

 of the lateral cell-row of the germ bands, ect {mikr.), which are descendants 

 of the micromeres. 



macromeres (Fig. 151 B). With the exception of this pro- 

 duction of micromeres at the animal pole, the anterior three 

 macromeres take no further part in the subsequent cleavage. 

 They contain the nutritive yolk of the egg^ and later supply 

 the cell material for the formation of the mid-gut ; they are 

 therefore to be designated as entoblasts. At the time when 

 the neuronephroblast divides into eight cells symmetrically 



