ON THE DEVELOPMENT OF NEBALIA. 411 



In sections through developing My sis Nusbaum (1887) 

 showed ectoderm cells separating the different parts of the 

 brain from each other. In my longitudinal sections through 

 the brain at this stage I have not been able to demonstrate 

 such cells, though fig. 48 shows the different parts of the 

 brain fairly well. 



In the optic lobes, and in both parts of the deutero- 

 cerebrum, thei'e are now to be seen fibres, as well as the two 

 kinds of nerve-cells. 



In the optic lobes the fibres are present in the posterior 

 half only. Fig. 44 shows a transverse section taken about 

 midway through the optic lobes. In it one can see the two 

 lateral optic ganglia, each slightly bilobed, and the central 

 mass, which is also bilobed, the lobing in both cases being 

 much more distinctly marked on the dorsal than on the 

 ventral surface. The median mass is, as I have said above, 

 in direct continuity with the anterior part of the deutero- 

 cerebrum. In this latter region it is widely bilobed on its 

 dorsal side, and transverse fibres can be seen passing between 

 its lateral parts (fig. 47). Fibres can also be traced from the 

 antennulary ganglion into the antennule (fig. 50, n.f.). 



The smaller nerve-cells are grouped very definitely and 

 symmetrically all through the brain, forming more or less 

 geometrical designs (figs. 44 — 46), but the main arrangement 

 of the nervous elements is that large ganglion cells lie 

 outside, within these smaller ganglion cells^ and within these 

 again, fibres (figs. 44 — 51). 



Behind the region shown in fig. 47 the anterior part of the 

 stomodEeum pushes up, as it were, between the two latero- 

 dorsal lobes of the central nervous mass, so that at the level 

 of the first antenna there are no more transverse fibres to be 

 seen, and the ganglia of the antennules appear to be in 

 direct continuity with the ventral and dorso-lateral portions 

 of the central mass, which here comes to an end (fig*. 49 and 

 50). As in the preceding stage, the tritocerebrum, which 

 consists merely of the pair of ganglia which innervate the 

 second antennae, is not nearly so large as either of the two 



