ON THE DEVELOPMENT OF NEBALIA. 417 



space between the central mass of ganglion cells and the 

 dorso-lateral mass on each side is spanned by fibres (fig. 61, 

 g. lY, also Claus 1889, fig. 3, Taf. ix). 



The ganglia of the first and second maxillae both consist, 

 at their widest parts, of a ventral mass of ganglion cells 

 which is continued dorso-laterally into two rounded masses of 

 cells, one on each side, the space between these masses being 

 spanned by fibres. In shape these maxillary ganglia are very 

 much like those of the adult, and, as in the adult, the fibres 

 from them to the maxillee go off between the dorso-lateral 

 humps on each side and the ventral mass. 



The first, second, and third thoracic ganglia are similar to 

 those of the maxillae. Between the ganglia there ruus a 

 double cord of nerve cells. The centre of each half of this 

 cord is filled with fibres (fig. 64, nf.). 



The posterior ganglia of the ventral chain are as yet tri- 

 angular masses of nerve cells, each triangle having a slightly 

 double appearance, and lying with its apex directed ventrally. 



One cannot at this stage speak with certainty as to the 

 future part to be played by the central mass of nervous tissue. 

 It seems not improbable that it furnishes fibres in the later 

 stages of brain development. 



As will be seen from the above I have not been able to find 

 a central mass in any of the ganglia of the ventral chain. 



Tiie Eyes. — The separation of the optic lobes from the 

 yolk, which, indeed, began in the more advanced specimens 

 of Stage E, is here almost complete. 



On the outer and lower edge of each optic lobe the outer 

 ectodermal layer can be seen in some places to be more than 

 one cell deep (figs. 59 and 60), and this leads me to believe 

 that this layer gives rise not only to the corneal hypodermis, 

 but also to the cone cells. 



The large cells with pale nuclei can be easily recognised 

 again here (figs. 58, 59, and 60, jj.?;.), though cell outlines can 

 now no longer be distinguished. Nor can one at this stage 

 see any connection between these large cells and those of the 

 optic ganglion. 



