514 STANLEY C. BALL 



their color is an intensely dark brown (fig. 30 e) . At low mag- 

 nifications the pigment cup is to all appearance black. 



The nucleus of the eye pigment cell is gradually forced by the 

 melanin granules against the membrane at the postero-median 

 side. Figure 32 shows it thus flattened. In proportion to the 

 size of the cell the nucleus is now smaller than during the period 

 of its greatest activity. 



6. Development of the brain and larger nerve trunks. Bresslau 

 ('04) determined that the formative mass in the embryo of 

 Mesostomum ehrenbergi soon separated into three anlagen, a 

 large antero-ventral mass, consisting at first of two ganglia which 

 later formed the brain; a spherical portion just posteriorly from 

 which the pharynx musculature arose; and third a smaller 

 cresentic gonad anlage. 



Hallez ('09) noted the similarity exhibited to this stage of 

 Mesostomum by the corresponding stage of Paravortex cardii. In 

 the development of P. gemellipara the differentiation of the brain, 

 pharynx musculature, and gonads is essentially the same as in 

 the species studied by Hallez. In two laterally symmetrical 

 regions toward the dorsal side of the anterior embryonic mass a 

 proliferation and grouping of nuclei occurs soon after gastru- 

 lation. Thus the two ganglia of the brain arise. While the 

 nuclei become more and more numerous, it has been impossible 

 to discover any mitoses in this region, a fact which led Hallez 

 to suggest that division here is amitotic. 



The second feature noted in the development of the brain is the 

 appearance simultaneously of two clearer protoplasmic areas in 

 the center of each lateral ganglion. The substance occupying 

 these two regions has been called Tunktsubstance' by Leydig. 

 As pointed out by Bresslau and Hallez, these two centers soon 

 fuse to form a transverse dumb-bell shaped core on the outside 

 of which the ganglion cell nuclei are arranged. The latter are 

 at first indistinguishable from those of the surrounding portions 

 of the undifferentiated cell mass, but in slightly older stages are 

 visibly smaller (figs. 29 and 53). 



Concerning the Tunktsubstance' mentioned above one char- 

 acteristic is to be noted that neither Bresslau or Hallez described 



