412 MARGARET ROBINSON. 



parts of the brain which lie in front of it (fig. 48, t.c). 

 These ganglia of the third pair are separated from each other 

 by the labrum (fig. 51). In some specimens here, as in the 

 first antennae, one can see fibres running from the ganglion 

 to the antenna. These fibres seem to run from the smaller 

 dark-staining nuclei. This fact, taken with the arrangement 

 of these small nuclei in the patterns alluded to above, leads 

 one to think that the fibres originate from these small nuclei. 



To sum up. The brain in this stage, as in the last, 

 consists of three pairs of ganglia and a central mass of 

 nervous tissue. This central mass extends farther forward 

 than it did in the last stage, and in it, and in each of the 

 ganglia, nerve-fibres have made their appearance. The deu- 

 terocerebrum consists of two parts lying one behind the 

 other, but the anterior of these parts is not the anterior 

 portion of the second pair of ganglia, but the posterior 

 portion of the central mass of nervous tissue mentioned 

 above. As has been stated above, this central mass does 

 not originate from the primitive ganglia first seen in the 

 embryo with nauplius appendages, but from the median band 

 of cells lying between the ganglia of the first and of the 

 second pairs. 



In the stage at present under consideration it extends 

 from almost the most anterior region of the space between 

 the optic ganglia to the place where its two lateral lobes join 

 the ganglia of the antennules. It consists now of two parts 

 — (a) lying between the optic ganglia, {h) lying behind this 

 and in front of the ganglia of the first pair of antenna). 



Between the antennary ganglion and that of the mandible 

 on either side there is a narrow chain of nerve cells, repre- 

 senting a future commissure. 



There is, as in the last stage, a deep groove between the 

 two ventral halves of the mandibular ganglion. In the 

 anterior part of this ganglion fibres can be seen in the 

 middle of each half (fig. 52 «./.). It is difiicult to say whether 

 the ganglion is formed by the fusion of merely the ganglia 

 of either side, or by the fusion of three elements, the ganglia 



