ANNELIDA 321 



placed at the posterior pole (Fig. 151 C), additional nuclei 

 make their appearance in the entoblasts without any corre- 

 sponding division of the entoblasts. In addition, however, 

 certain cells that are from the beginning distinct are con- 

 stricted off from the entoblasts; they lie under the layer of 

 micromeres, and are to be looked upon as the earliest 

 entoderm cells. Later, cells that have been differentiated 

 within the entoblasts are added to them, so that a distinc- 

 tion between the two kinds can no longer be made. 



The embryo, up to the stage to which we have followed it, 

 consists of a solid mass of cells formed of the three macrO' 

 meres (entoblasts), which become partly covered over by the 

 disc of micromeres, which have now become very numerous 

 (Fig. 151 C). Under the latter, consequently between them 

 and the entoblasts, there already lie a number of entoderm 

 cells, while the nuclei that appear within the entoblasts 

 provide for the formation of further cell material to be 

 added to the entoderm. At the posterior pole appear the 

 two symmetrical groups of neuronephroblasts, each com- 

 posed of four large cells (Fig. 151 C), and below them, 

 sunk somewhat deeper, lie the two mesoblasts ; these, too, 

 are now almost symmetrically arranged, although that re- 

 lation cannot be recognized in a surface view, such as 

 Fig. 151 C. 



The two groups of five cells each at the posterior end are 

 of great importance from the fact that the greater part of 

 the body of the leech arises from them. Since in their origin 

 they can be traced back to the hindermost of the four 

 oi'iginal macromeres, it follows that this is the one which is 

 responsible for the development of by far the largest part 

 of the body. These two groups of cells undergo the follow- 

 ing change : from the anterior face of each of the ten cells 

 new cells are constricted off by repeated cell division, a pro- 

 cess which can be compared with the multiplication of the 

 pole cells of the mesoderm in the Chaetopoda, and also leads 

 to the same result. On each side, then, there arise four 

 adjacent rows of cells, those of the neuronephroblasts, and 

 one lying somewhat deeper, that of the mesoblasts. All of 

 them together constitute the two germ bands, which, how- 



K. H. E. Y 



