ON THE DEVELOPMENT OP NEBALTA. 401 



first rudiment of the optic ganglion. They are continuous 

 with similar cells lying near the base of the first antenna, 

 and forming the rudiment of the antennulary ganglion (fig. 

 20, g. ii). At the point of junction of these two rudiments 

 there is a distinct groove or fold in the cells composing them 

 (seen in transverse section, fig. 19, and in longitudinal 

 section, fig. 23, between g. i and g. ii). This fold makes it 

 appear as though the optic ganglion cells were being nipped 

 oft' anteriorly and laterally from those of the antennulary 

 ganglion, these last lying nearer the middle line. Now, it 

 might be argued that there is here a case of the optic 

 ganglion being budded off from the antennulary ganglion ; 

 but one must bear in mind that these rudimentary ganglion 

 cells are at present merely slightly differentiated ectoderm 

 cells which still form part of the outer wall of the body of 

 the embryo, and also that all the evidence we have seems to 

 show that the differentiation has taken place simultaneously 

 in front of, and just behind the groove, and in the cells of 

 the groove itself. This groove or fold marks the division 

 between the optic region and that of the first antenna. It is 

 in fact almost the earliest trace of segmentation. 



There are similar groups of specialised ectoderm cells near 

 the bases of the second pair of antennas and of the mandibles 

 figs. 21 and 22, g. iii, g. iv). 



There are then in this embryo with nauplius appendages 

 four pairs of rudimentary ganglia — (1) optic, (2) antennu- 

 lary, (3) antennary, (4) mandibular (fig. 23). 



Since the optic ganglia appear simultaneously with the 

 other pairs of ganglia but independently of them ; since too 

 experiments made by Herbst (1896) and by others before 

 him have shown that eye-stalks in Decapods may be replaced 

 by antenna-like appendages, the optic region possesses two 

 of the main essentials of a segment, and there seems to be 

 little reason for not considering- it the first segment of the 

 body. 



It was on account of the replacement of the eye-stalk of 

 Palinurus by an antenniform palp that Milne-Edwards, 



