ON THE DEVELOPMENT OF NEBALIA. 405 



marked pair of ganglionic swellings, and the first three of 

 these pairs are now (since the second antenna and its ganglion 

 have moved forward) joined to help in forming the syncere- 

 brum. Of these three pairs of swellings those in the optic 

 and antennulary segments are much larger than those in the 

 segment bearing the second antennas. Each ganglion now 

 consists of two kinds of cells which may be distinguished by 

 their nuclei. 



(a) Cells with large clear nuclei, each nucleus being sur- 

 rounded by an appreciable quantity of protoplasm, though 

 cell outlines can now no longer be made out. These cells are 

 seen to divide, hence it may be reasonably inferred that they 

 give rise to — 



(b) Cells with small darkly-staining nuclei. These lie for 

 the most part on the inner side of the large cells. The small 

 nuclei are closely packed together, and the protoplasm sur- 

 rounding each of them is very small in amount, almost in- 

 appreciable. Some of these small nuclei are to be seen in 

 each of the three pairs of appendages behind the eyes (first 

 and second antennse and mandibles) as well a§ in the ganglia. 



The rudimentary brain here consists mainly of three pairs 

 of ganglia, and may be spoken of as consisting of three parts 

 — the proto- deutero- and trito-cerebrum. 



In their most anterior region the two optic ganglia are 

 separated, as they were in the last stage, by a narrow band 

 of ectoderm cells only one cell deep (fig. 29), but posteriorly 

 there lies between the optic ganglia a central mass of nervous 

 cells which, from the appearance of the section shown in 

 fig. 31 and others, seems to have arisen by an invagination 

 from the cells of the band of ectoderm which, in the preceding 

 stage, separated the ganglia of the first pair in this region. 



In his work on the Crayfish, while describing the state of 

 the nervous system in an embryo which is a little more ad- 

 vanced than this, Eeichenbach (1886) figures a central mass 

 lying between the two optic ganglia. This central mass con- 

 sists of three parts, two large outer ones which he considers 

 to be parts of the supra-cesophageal ganglion of either side 



