154 CHARLES R. STOCKARD 



such an occurrence is simply accidental and due to the fact that 

 the specimen was already arrested or defective in an early stage 

 as might by chance happen in any normal lot of eggs. It is 

 clearly true, as I shall show beyond, that only very early and 

 carefully regulated treatment can artificially produce twins and 

 double monsters, a phenomenon which must happen about the 

 stage of gastrulation. Therefore, the treatment must be applied 

 much before this time. Cyclopia may be induced by slightly 

 later treatments, but only during a rather limited time, and 

 quite early at that. 



Other experiments will later be considered in order to illustrate 

 the difference in response on the part of these eggs following 

 treatments similar to those above, but applied as nearly as pos- 

 sible at certain particular developmental periods. 



d. Differences in effect between greatly reducing the developmental 

 rate and actually stopping temporarily the process 



In the foregoing review of experiments attention was fre- 

 quently called to the fact that in certain of the low temperatures 

 employed an almost complete stop in development was actually 

 obtained, while at the somewhat higher degrees the progress of 

 development was reduced to an extremely slow rate, but not 

 actually stopped. A more specific comparison between the ef- 

 fects resulting from actually stopping and greatly slowing the 

 rate of development may now be made. 



Three groups of Fundulus eggs when in the two-cell stage were 

 placed in temperatures of 5°, 7°, and 9°C., respectively, as 

 reviewed under experiment 901, B series. The first two temper- 

 atures wxre sufficiently low to almost completely stop develop- 

 ment, so that after twenty-four hours of such exposure the eggs 

 were still in the two- or four-cell stage. The group at 9°C., 

 however, developed very slowly and attained either sixteen- or 

 thirty-two-cell stages within the first twenty-four hours. In 

 other words, at this temperature three or four cell divisions occur 

 per day. When all had remained for three days in these low tem- 

 peratures, they were removed from the refrigerator and the fol- 

 lowing results ensued: 



