140 CHARLES R. STOCKARD 



yolk-sphere. The inhibition resulting from the cool nights of the 

 early season very probably accounts for the almost uniform inferiority 

 of emljrj^os developed at this time as compared with those developing 

 during early July, the height of the spawning season for thus locality. 



Lots Bi and B2, in temperatures of 5° and 7°C., respectively, for 19 

 hours, were all in either 2- or 4-cell stages. They were thus almost 

 completely stopped in development. The 2-cell stage was about 

 reached when they were placed in the low temperatures, and probabl}' 

 some were dividing the second time before the surrounding water had 

 cooled to the temperature of the refrigerator (all dishes contained 60 

 cc. of sea-water). 



Lot B3 at 9°C. contained after 19 hours fairly regular 16- and 32-cell 

 stages. At this temperature cell division had been able to continue, 

 although at a greatly reduced rate, accompHshing only three or four 

 divisions in the 19 hours. 



The control eggs 48 hours after fertilization showed the germ-ring 

 onl}^ one-quarter over the 3^olk sphere, with the embryonic shield be- 

 ginning to form (fig. 4), a stage that should be attained within 24 

 hours during the warmer part of the season. 



Lot Bi, after 45 hours at 5°C., was in first-, second-, or third-cleavage 

 stages. The arrangement of the cell groups was often very irregular 

 and many cells contained large vacuoles. There were a very few 

 almost typical 2- and 4-cell groups. In some of the '2-cells' a large 

 central vacuole seemed to ahnost divide each of the cells (fig. 5) . 



These eggs at 5°C. have thus only in rare cases divided more than 

 once during 48 hours. This lot was now removed from the refrigerator 

 and returned to the room temperature after being 45 hours in the cold. 



Lot B2, at 7°C., was in much the same condition as lot Bi, except 

 that some eggs had undergone one or two further cleavages. There 

 were many irregular cleavage patterns and a few almost regular 16- 

 or 32-cell stages. A number of the germ-discs consisted of irregular 

 partly divided masses (fig. 6). 



Lot B3, at 9°C., had developed very slowly but fairly well, and now 

 after 45 hours in the low temperature contained germ-discs composed 

 of from 64 to about 128 cells. The cell arrangements and shapes of the 

 discs were almost uniformly regular. Therefore, at this temperature 

 development progresses, though very slowly, and none of the cell masses 

 had yet begun to flatten down to cap the yolk-sphere. 



When 3 daj^s old, the control embryos were well formed, although 

 the germ-ring was not yet entirely over the yolk-sphere, much the 

 same stage as shown above in figure 1. 



Lot Bi, after being at room temperature for 24 hours, had passed 

 from the 2-, 4-, and 8-celled conditions and had reached a high seg- 

 mentation stage. The discs had not fully fiattened on the yolk- 

 spheres, but were beginning to descend. There was no gross indication 

 of germ-ring or embryonic-shield formation. Many eggs had promptly 

 recovered their ability to develop on return to higher temperature and 

 had progressed during the 24 hours about as far as the control had gone 

 during the first 24 hours of their development. 



