ON THE DEVELOPMENT OF NEBALIA. 407 



second antenna and the mouth, disappears altogether (figs. 

 38, 39). 



Sumniiug up our knowledge of the brain at this stage one 

 may say that it consists of three pairs of lateral ganglia and 

 a median mass of nervous tissue which extends from the pos- 

 terior region of the first pair of ganglia, where it begins as a 

 double cord, to just in front of the third pair of ganglia, 

 where it euds as a single cord. 



The ganglia of the mandibles and first maxillse at this 

 stage are much spread out and flattened (figs. 40, 41), while 

 the second maxilla has the merest rudiment of a ganglion. 



The Eyes. — Sections through the anterior I'egions of the 

 optic lobes show a slight depression in the ectoderm cells 

 forming the outer wall (figs. 28, 29, d.). These cells are pro- 

 bably the forerunners of the crystalline cone cells or of the 

 cells of the corneal hypodermis, or, as I believe, of both these 

 sets of cells. Very slightly behind this depression lies the 

 anterior limit of the optic ganglion, and on a level with this 

 the optic invagination (fig. 30, op. in.). This has now the form 

 of a solid cup. On the inner side of this now solid invagina- 

 tion there can be seen a few of the smaller nerve nuclei. 

 Reicheubach (1886), in his account of the development of the 

 eye in the Crayfish, says that the invagination first becomes 

 solid and then divides into two layers, the inner of which 

 furnishes cells to the optic ganglion, while the cells derived 

 from the outer layer become retinula3. The state of things 

 shown in fig. 30 is very like that described and figured by 

 him in a Crayfish embryo at about this stage. 



Other Ectodermal Structures. — The stomodteum still 

 runs forward from the mouth, and in this stage there is the 

 fii'st appearance of the anus and proctodseum (fig. 42, ijroc). 

 In a transverse section through the caudal papilla of an 

 embryo with nauplius appendages I noticed two exceptionally' 

 large ectoderm cells. In this stage also I find two very large 

 cells in similar sections just behind the anus (fig. 43, I.e.), but 

 I have not succeeded in tracing these cells in later stages. 



Nusbaum (1887) found similar cells in a very early stage in 



