DEVELOPMENT OF PARAVORTEX GEMELLIPARA 515 



in either text or figures, namely, the striated or fibidllar structure 

 of the lighter brain core when viewed in sagittal sections. That 

 these striations are to be regarded as the fibers or prolongations 

 of the ganglion cells whose nuclei are distributed over the surface 

 of the brain, is clear from the works of many investigators. These 

 structures are well represented in Bronn's Klassen und Ordnungen 

 des Thier-Reichs. Obviously it is these fibers which, in a cross- 

 section of the brain, i.e., a sagittal section of the worm, are 

 responsible for the dotted appearance. Figures 29 and 37 are 

 of sections so cut that some nerve fibers are seen in cross and 

 others in longitudinal section. 



Little remains to be added in regard to the nervous system. 

 Since the material was not treated with differential nerve stains, 

 it is extremely difficult to follow the nerve trunks through the 

 mesenchyme, even in older specimens. 



Concerning the origin of the two posterior nerves of Para- 

 vortex cardii Hallez says: 



''Quant aux deux troncs nerveux posterieurs, tout ce que 

 j'ai pu voir, c'est qu'ils se differencient in loco. . . ." 



It is believed that in P. gemellipara these posterior nerves 

 can hardly be said to arise 'in loco.' They evidently push back- 

 ward through the mesenchyme from the posterior angles of the 

 brain. In figure 28 the nerve fibers can be traced thus for a 

 very short distance, but in a later stage (fig. 29) the ends have 

 reached farther toward the posterior end. 



It is important to notice that many ganglion cell nuclei are 

 carried backward along the nerve trunks. Consequently the 

 cephalic mass suffers a reduction. From the conditions observed 

 it is concluded that in the Turbellaria, as in the vertebrates, 

 the nerve fibers, with groups of ganglion cells, originate in the 

 region of the brain and migrate toward the periphery. 



That the optic nerve trunks have a similar origin may be 

 argued from the appearance presented by figures 28 and 37. 

 However, these nerves may have another origin. The fibrous 

 brain core in figure 28 has begun to stretch forward at two antero- 

 lateral points; a decided advance has taken place at the stage 

 pictured in figure 32. Here the left lobe of the brain and the 



JOURNAL OF MORPHOLOGY, VOL. 27, NO. 2 



