ON THE DEVELOPMENT OF NEBALTA. 413 



of either side, and a median strand. I incline to think that 

 it is formed from the fusion of two ganglia only, though the 

 central part looks very like the central mass described above, 

 and Reichenbach describes a median strand as forming part 

 of the ventral chain in the Crayfish. 



Posteriorly the mandibular ganglion is flattened and spread 

 out as it was in the last stage, but the ganglion of the first 

 maxilla has now assumed a more compact shape. It has no 

 fibres as yet. 



The Eyes. — Fig. 46 shows the optic invagination fairly 

 well. Other sections in the same series show a closer 

 contiguity between the cells with large colourless nuclei and 

 the small dark-staining nuclei of the nerve-cells which here 

 abut on them. It seems not improbable that these small 

 nerve-cells have originated from the optic invagination, 

 though I can find no more definite indication of it than that 

 shown in fig. 30. 



In contra-distinction to the other cells at this stage the 

 cells of the outer wall of the invagination show distinct cell 

 outlines. Their nuclei, too, are larger and paler than those 

 of the other cells. 



Other Ectodermal Structures. — The labrum (fig. 52 

 and fig. E)is much larger than it was in Stage D, and in some 

 specimens there is a great advance in the growth of the 

 stomodseum, the future stomach being distinctly foreshadowed 

 (figs. 53, b4st.). 



The lateral ectodermal thickenings (future shell valves) 

 have increased in size (figs. 51 and 53). In one or two 

 specimens I have found a distinct median dorsal thickening 

 of the ectoderm. This I take to be the first stage in growth 

 of the dorsal part of the bi- valve shell (figs. 53, 54 s.). 



Mesoderm. — The mesoderm cells have increased in 

 number, and are now pretty evenly distributed along the 

 whole length of the embryo, i. e. they are now no longer 

 more abundant in the papilla than elsewhere. Fig. 54 shows 

 several mesoderm cells lying round the stomodaeum, some of 

 them very closely attached to it. 



