CELL CONSTANCY IN THE GENUS EORHYNCHUS 279 



are so closely packed together that even in a most careful recon- 

 struction from thin serial sections one is never absolutely certain 

 that in some cases two adjoining sections do not contain parts 

 of the same nucleus. Reconstructions of two individuals from 

 drawings of serial sections magnified nine hundred diameters gave 

 in one instance one hundred and eight cells in the brain, in the other 

 one hundred and nine. While these facts are not given with the 

 idea of establishing an absolute constancy they may be taken to 

 indicate that, evien though the structure in this region is so ex- 

 tremely crowded, yet there is a close agreement in the results 

 obtained from these two reconstructions. 



Eorhynchus emydis. A reconstruction of the brain of a single 

 individual of this species gave ninety-four nuclei as the total for 

 that structure. 



12. The genital ganglion 



Eorhynchus gracilisentis. Eighteen cells are grouped together 

 in the region of the cirrus in the male to form what is commonly 

 spoken of as the genital ganglion (fig. 17 gg). As in the brain, 

 these cells are all of the same type, giving no basis for a division 

 of the ganglion into regions according to the structure of the com- 

 ponent elements. 



13. Body musculature 



Eorhynchus graciUsentis. Two reconstructions of the body 

 musculatiu-e each gave a total of thirty nuclei associated with that 

 tissue. These were arranged in fifteen pairs, extending from the 

 union of the body with the proboscis, caudad to the posterior ex- 

 tremity, one nucleus of each pair having a position slightly lateral 

 to the dorsal canal. The distribution is such that a single cross sec- 

 tion could not contain more than a single pair of nuclei. The 

 difficulties involved in the determination of the number of nuclei 

 present in this tissue consist chiefly in the discrimination between 

 these nuclei and the embryos, which, in the female, are often so 

 closely packed against the wall that only the most careful study 

 will reveal whether a given structure is associated with the muscle 

 layer or merely closely applied to it. The position of these nuclei 

 in longitudinal section is shown in figure 5. 



