DEVELOPMENT IN PLANARIANS. 2/3 



do not seem at this period to arise from the surface epithelium. 

 At the time of the formation of the anlage of the permanent 

 pharynx, cells like these in a corresponding region may be seen 

 multiplying by direct division (Fig. 7, //). When the brain 

 anlages first become clear, owing to the formation of nerve fibers, 

 similar cells may be seen lying among the fibers (Fig. 7, c]. In 

 the nerve cords, as they grow toward the tail region, correspond- 

 ing nuclei may be seen, often in columns and often appearing to 

 divide by direct division. It seems possible, therefore, that the 

 nerve cells of the central nerve system of these planarians may 

 arise by direct division from neuroblasts which are differentiated 

 in the anterior ventral wall of the embryo at a comparatively early 

 period, and that the other cells of the central nervous system may 

 arise by direct division from these. 



The cells of the peripheral nerve plexus may arise, however, 

 in the periphery and not from those of the central nervous 

 system. In the adult many very pale nuclei may be seen 

 scattered here and there below the musculature. These nuclei 

 resemble those of the cells found within the central nervous 

 system. 



During embryonic development, therefore, we find that the sur- 

 face epithelial cells are derived by direct division from the pre- 

 existing ectoderm cells. The intestinal cells certainly, and also 

 probably the nerve cells, multiply by indirect division. Whether 

 additions are made from parenchyma cells after the primary 

 period of differentiation is uncertain. The early myoblasts multiply 

 partly, at least, by indirect division. The period at which the 

 differentiation of myoblasts from the parenchyma ceases is unde- 

 termined. Branched, anastomosing connective-tissue cells, " leu- 

 cocytes," rhabdite cells and gland cells are differentiated in the 

 parenchyma but certain cells retain a size and form characteristic 

 of an early stage of embryonic development. 



REGENERATION IN EMBRYOS. 



When removed from the egg-capsule embryos of the stages 

 represented in Fig. i, A and B, and younger ones live but a short 

 time in water. Twenty-four hours is the longest I have been 

 able to keep them alive. Embryos of the stage shown in Fig. 



