^528 H . H. Newman 



only on the application of some stimulus, such as squirting 

 them violently out of a pipette or pricking the egg membrane 

 with a sharp needle. Such specimens lack the initiative to hatch 

 v^ithout some assistance and would have died within the egg mem- 

 brane had not assistance of some sort been given them. When 

 released these individuals swim about rather sluggishly, and, 

 on being transferred to aquaria are very apt to float on the surface 

 and allow themselves to be drawn by the currents to the gauze 

 around the standpipe, and thus perish. Active young fish, such 

 as the previously hatched H hybrids, always go to the bottom and, 

 if drawn by a current, swim actively away from the region of 

 danger. 



Figs. I and 2 show comparative sizes of M pure and .1/ hybrid embryos at nineteen days (Series I). 

 Fig. I represents an average specimen of M hybrid. 

 Fig. 2 represents an average specimen of M pure. 



The figures are camera drawings of embryos as they lay within the egg membrane, quieted with 

 chloretone. Figures show a magnification of 12 diameters. 



At 19 days: No more H hybrids hatched, but many still living. 

 The M pures had grown considerably and looked as though they 

 were almost ready to hatch. A fairly heavy deposit of grayish 

 pigment covered the body but was sparingly scattered on the yolk 

 mass. The M hybrids showed no change except that they had 

 grown darker. Outline camera drawings of typical embryos of 

 the two majalis strains at this stage will emphasize the very marked 

 difference between them (Figs, i and 2 ). 



At 22 days: About twenty M pures hatched. A typical speci- 

 men, drawn on hatching, is shown in Plate V, Fig. M. This 

 specimen was 1 1 mm. in length as compared with 7.2 mm. for the H 



