172 F. X. WILLIAMS 



have this character of periodicity less pronounced than the adult. 

 Some of these larvae on being placed at 11 a.m. into a dark room, 

 emerged from their hiding places, and while not glowing with 

 their usual brilliancy, walked about and fed as if the evening were 

 at hand. 



NOTES ON THE EMBRYOLOGY OF PHOTURIS PENNSYLVANICUS 



The egg (fig. 31) is subspherical, pale lemon yellow, and 

 measures about 784 by 677 micra in diameters. It has three 

 envelopes; first, a thin outer membrane which encloses many fine 

 oil-like drops and gives the surface a granular aspect; second, 

 the chorion, the stoutest and firmest of the envelopes; and third, 

 the vitelline membrane, a secretion of the egg itself. The dor- 

 sal and ventral sides of the egg appear similar, but the longer 

 axis evidently marks the anterior and posterior poles. I could 

 find but a single micropyle, and this was to one side of the pre- 

 sumably anterior pole. 



I have no stage which shows the formation of the blastoderm, 

 or peripheral cell layer. Figure 32 is a section through an early 

 blastoderm stage. No cell walls are visible, the cells being 

 flattened against the vitelline membrane on the one hand, and 

 on the other they bulge inwardly. The vitellophags or yolk 

 cells (V, fig. 32, and fig. 42) are considerably larger than the 

 blastoderm cells, and are often amoeboid or stellate. They are 

 not very abundant in the stage figured, but later they increase 

 in number, and doubtless, through the agency of protoplasmic 

 processes, form an extensive network. 



Embryonic envelopes. The amnion and the serosa appear be- 

 fore there is any indication of a gastrula. This is clear from 

 figure 33, where the amnion, A, is thicker than the more flat- 

 tened serosa, B, and merges almost imperceptibly into the some- 

 what thicker germ-band proper, G. A dorsal view of the embryo 

 at this stage is shown in figure 34. It is about 245 micra long 

 and is thickly almond-shaped, depressed, the broad blunt end 

 being the cephalic one. The paler diamond-shaped area repre- 

 sents the point of contact between the amnion and the serosa. 



