The Early Development of Ligyda with 

 Reference to the Nervous System 



William A. Hilton 



The material for this study was obtained at Laguna Beach dur- 

 ing the summer of 1913. This species of isopod, Ligyda occi- 

 dentalis Dana, is very abundant on the rocks, and it was an easy 

 matter to secure females carrying the eggs or young, and any stage 

 could be easily obtained even late in the summer. With the fresh 

 specimens one of the most valuable methods for preliminary ob- 

 servation was to examine the embryos after fixation, but before dis- 

 section or staining or even changing to an alcoholic fluid. The use 

 of Gilson's fluid for this purpose made it possible to detect at once 

 the general features of the embryo even in early stages. This 

 reagent whitened all parts of the embryo and they could be seen 

 as opaque white on the yellowish yolk. Figures 1 to 21 were drawn 

 from embryos treated in this way. The disadvantages of study 

 by this method alone are: First, it is rather hard to tell the mean- 

 ing of certain parts which show, and, second, cell outlines are not 

 clear except in very early stages. Embryos fixed In this and other 

 ways were removed from the yolk, stained and mounted as a whole, 

 or cut in serial sections. 



In the early development of the embryo there Is a gradual con- 

 centration of cells at one pole of the egg. From Gilson's prepara- 

 tions it was learned that at a very early stage the general area of 

 the optic lobes was outlined, as shown in Figs. 1, 2, 3, and 4. 

 A white spot at the center of the area, later at the caudal end. Indi- 

 cates a thickening of cells which may be in the region of invagina- 

 tion. Gradually thickenings back of the optic area begin to Indicate 

 the position of the appendages; two of these are shown in Fig. 3, 

 and a larger number in Figs. 4 and 5. These first thickenings are 

 so faint and so variable in early stages that I cannot place much 

 value upon them. It seems probable, however, that the three 

 marked areas in the cephalic region of Fig. 5, represent the three 

 naupllar appendagel recognized by Nusbaum and others. In Fig. 



