166 G. E. COGHILL 



be regarded as trustworthy and of positive value. The other two 

 specimens may be regarded as corroborative evidence with refer- 

 ence to the count and as valuable illustrations of the changes that 

 are taking place in the distribution of the giant ganglion cells, 

 particularly in the tail region, during this period of growth. 



It is obvious from a glance at the graphic projections that there 

 is in the youngest embryo of the series a quite regular distri- 

 bution of the giant ganglion cells throughout the trunk region, 

 excepting through the level of the first three or four myotomes. 

 In the caudal region, on the other hand, they are thickly con- 

 gregated throughout the extent of the segmented mesoderm 

 and then more scattered, caudad. In comparison with this latter 

 feature, the embryo of the early-flexure stage shows a more gen- 

 eral scattering of the neurones through the cord of the lengthening 

 tail bud, and this same process of distribution is seen in the two 

 later stages. In the embryo of the coiled-stage the distribution 

 is quite regular through the cord caudad to the level of the twenty- 

 ninth myotome. This is the case, also, in the embryo of the early- 

 swimming stage. Caudad of this in the coiled stage the cells 

 are widely scattered through the level of the segmented meso- 

 derm. In the embryo of the swimming stage, on the other hand, 

 the cells are not distributed through the entire extent of the 

 segmented mesoderm. 



It seems obvious from this study of the numbers and positions 

 of the giant ganglion cells that there can be little if any prolifer- 

 ation of cells belonging to this series during the period under 

 consideration, and that the distribution of the cells through the 

 caudal portion of the cord is brought about chiefly or wholly 

 through mechanical processes allied with the differentiation of 

 the mesoderm, and after the cells of the system have become dif- 

 ferentiated into neuroblasts and neurones. The slight increase 

 in numbers from 210 to 253 on one side should be regarded as due, 

 not to proliferation of cells of the group, but to a progressive 

 differentiation which enables the observer to recognize more of 

 the cells with certainty. More detailed evidence on this point will 

 be given in following paragraphs. 



