164 G. E. COGHILL 



of comparative neurology, chiefly from the purely morpholog- 

 ical point of view. Questions of homology, however, have no 

 critical consideration in this paper. It is the relation of the giant 

 ganglion cells to a functioning and at the same time differentiat- 

 ing nervous system that is here the center of interest. It is not 

 the intention, however, to ignore the anatomical results of earlier 

 authors on the subject, and a consideration of their results will 

 be given farther on in the paper from the point of view of my own 

 findings, both . anatomical and physiological. 



The following anatomical observations have been made by 

 exhaustive study with the oil inamersion lens systems (Zeiss and 

 Bausch and Lomb) of serial sections of fifty-nine embryos selected 

 from a collection of over two hundred such series because of 

 their special adaptability to this phase of the problem. Of this 

 number, thirteen are of the non-motile stage; seventeen, of the 

 early-flexure stage; fourteen, of the coil-stage; ten, of the early- 

 swimming stage. As representative of these neurones in the 

 embryos of the non-motile stage over two hundred of the giant 

 ganglion cells and fibers have been studied minutely in their 

 various relations; of the early-flexure stage, about the same num- 

 ber; of the coil-reaction stage, approximately one hundred; and 

 about an equal number of the early-swimming stage. 



a. The distribution of the cells 



The distribution of the giant ganglion cells has been studied 

 exhaustively in one type specimen of each of the four stages of 

 development, and their positions have been charted in graphic 

 projections, on the vertical plane as figures 56 to 59 of this paper. 

 Exact counts have been made of the cells in these four cases only, 

 but my study of numerous embryos convinces me that the num- 

 ber of the cells and their manner of distribution is fairly constant 

 for each stage and that these features of the system, with minor 

 qualifications as noted farther on, are typically represented in the 

 figures cited. 



The cells charted in the figures 56 to 59 are on one side of the 

 median plane only, and, therefore, represent presumably about 



