488 J. T. GliADON. 



find in otlier embryos some interesting variations which 

 assist us very materially in gaining a fuller comprehension of 

 the subject. 



Fig. 15, which represents a longitudinal section of the 

 optic nerve of an embryo mouse of fourteen days, gives us a 

 better idea of the longitudinal fibrils than we have been able 

 to gain in considering the later stages of these fibrils in the 

 frog, as they lie in the same plane for a much greater distance, 

 and are, at the same time, rather thicker for a greater part 

 of their length than those we have seen in the frog. 



On the other hand, in fig. 16, representing a longitudinal 

 section of the optic nerve of a developing trout (length of 

 optic stalk '5 mm.), we see fibrils that are extraordinarily 

 thick, and consequently very easily seen near their nuclei, 

 but undulating to such an extent that it is only possible 

 to follow thein a very short distance away from their nuclei. 



In fig. 17 1 have shown a transverse section of the optic 

 nerve of a 33 mm. dog-fish, and on p. 486 I have compared 

 the nuclear outlines of the densely packed optic nerve of the 

 frog (fig. 8) with those of the much less densely packed optic 

 nerve of the dog-fish, shown in this figure, so that I need 

 only point to the difi"erence in the arrangement of the nuclei 

 themselves, though, as I have stated on p. 485, the stellate 

 arrangement of the nuclei and the trabeculte in the optic 

 nerve of the tadpole is lost before the adult stage is reached. 



Another point of difference between the frog and the dog- 

 fish lies in the fact that the pial sheath of the latter is richly 

 supplied with blood-vessels, though not represented in the 

 drawing. 



A transverse section of the optic nerve of an eight-day 

 chick (fig. 18) shows certain peculiarities of trabecular forma- 

 tion ; the arrangement of the nuclei is free, like that of the 

 dog-fish, but the nuclear outlines more closely resemble those 

 of the frog, due, in my opinion, to the same causes as those 

 I have given to account for the peculiarities of these outlines 

 in the frog. 



I have shown one of the numerous capillaries that supply 



