NOTES ON THE EMBRYOLOGY OP LIMULUS. 
541 
adding to the substance of the nervous system. I am not 
positive as to the origin of the commissures. From the figure 
last cited it would seem that here as in many other Arthropods 
they arose as epiblastic ingrowths. This at least is most 
reasonable. In the stage of fig. 12 (fig. 29) there are distinctly 
eight pairs of postoral ganglia and this number is not exceeded 
in embryonic life. Of these six correspond to the six pairs of 
ambulatory appendages, one to the operculum and one to the 
first pair of respiratory appendages. As will be seen, there is 
no ganglion for the metastoma (chilaria), another proof, if 
more were necessary, that they are not to be regarded as 
morphological equivalents of the limbs. 
In the stage of fig. 14 several important features are intro- 
duced. Here begins that concentration of the anterior part of 
the nervous system which results in the nervous collar of the 
adult. I have found it difficult to trace the changes and have 
obtained my best results by external views of the whole animal 
treated in the following manner : — On removing the embryo 
from its envelope I placed it in a watch crystal on the stage of 
the microscope and then added some fifty per cent, alcohol. 
This, on penetrating the cuticle, first affected appreciably the 
protoplasm of the nervous centres rendering it alabaster white, 
and thus in most regions readily distinguishable against the 
darker background formed by the as yet indifferentiated food 
yolk. Soon, however, other parts were affected and their dis- 
tinctness was lost. Fig. 14 was drawn from these specimens 
so treated, and as a camera was used it is correct in all 
respects except possibly the brain and the metastoma. These 
parts I was unable to see distinctly, and from my sections I 
am inclined to believe them incorrect. This figure shows pos- 
teriorly the nerves going from the respective ganglia to the 
sixth, fifth, and fourth pairs of legs. In the third and second 
pairs of nerves there seems to be a shifting forwards so that 
they do not arise exactly opposite to the centres of the ganglia 
but rather from their anterior margins. I have shown on a 
preceding page the manner of the shifting of the mouth, and 
this change in its position, and, consequently, in that of the limbs 
