STRUCTURE AND DEVELOPMENTAL RATE 147 



The above 6 unhatched embryos in lot B2 were kept in order to deter- 

 mine how long such specimens might be able to survive. When 52 

 days old, these specimens were still alive, although the yolk-sphere 

 had become very small, being almost absorbed. The small monsters 

 were practically at a stand-still as to their life processes and were not 

 kept after this time. 



These experiments are here considered in a general way with-] 

 out going into the details of the deformities concerned. Theyl 

 demonstrate the fact that a normally continuous development' 

 may be modified into a discontinuous one by stopping its course 

 during a very early cleavage stage. The fact is also shown that 

 this stoppage is followed by a too slow resumption of the devel- 

 opmental rate and results in about 33 per cent of gross anoma- 

 lies among those specimens able to survive the treatment. The 

 mortality induced by stopping at such periods is high, the major- 

 ity of eggs in all cases dying after return to normal temperature. 

 Great variation in ability to withstand such treatment is shown 

 by these hardy Fundulus eggs. The weakest ones succumb with- 

 out resuming development on removal from the cold. Stronger 

 specimens may undergo a few^ further divisions and live for some 

 time in a high segmentation stage without being able to continue 

 or progress further in their development. Other eggs continue 

 development, but in such extremely abnormal fashion as to fail 

 completely to form the embryonic body and only differentiate 

 certain tissues scattered irregularly over the yolk-sac. Still more 

 hardy specimens succeed in forming the embryonic body, but 

 many organs requiring a high degree of cell proliferation and 

 growth for their development, such as the eyes, other brain diver- 

 ticula, mandibular, hyoid, and branchial pouches, etc., are unable 

 to form in a normal fashion, and numerous defects in these parts 

 are to be found. 



Finally, the most resistant or hardiest eggs withstand the stop- 

 page due to the low temperature and are able to resume develop- 

 ment at an almost normal, though slightly retarded rate. These 

 individuals may seem typically normal in structure, and often 

 develop into hatched free-swimming fish, yet even these not in- 

 frequently show some indication of a subnormal condition in 

 having their bodies slightly twisted or bent, and in being unable 



