170 CHARLES R. STOCKARD 



which had occurred. The control consisted of 114 eggs, each of which 

 contained a normal well-formed fish. In the Ai lot 4 more had died, 

 and thus the total mortality in this group after removal from the cold 

 was very high, a little over 70 per cent. In all 54 individuals had 

 survived to develop embryos, and of these 16, or 30 per cent, showed 

 gross abnormalities. Five of the 16 abnormal ones showed double condi- 

 tions. One was a complete twin, two were double-headed and two 

 had double anterior halves with single tails, Y embryos. Thus 9.3 per 

 cent of all surviving embryos were specimens exhibiting some degree of 

 doubleness, and 33 per cent of the deformities which occurred were 

 duplicities. When we consider the very delicate degree of arrest and 

 the particular developmental moment that must be affected on the 

 basis of our explanation of double monsters, the above result is a remark- 

 ably significant one and is as good as any I have obtained by this 

 method during the past three seasons. 



In the A2 group at 10 days old 2 others had died and 88 were now 

 alive. Among the 88 survivors eleven individuals, or 12.5 per cent of 

 all, were grossly deformed and many others were pale in color and far 

 behind the average in their degree of development. Two of the 11 

 grossly deformed specimens were double, one showed a slight degree 

 of anterior duplicity and the other was a twin with the two embryos 

 180° apart on the yolk. One of the twin components was large, well 

 developed and normal in structure, the other was a short embryo with 

 almost no body but with a well-formed head containing eyes and a 

 pulsating heart and good blood circulation. In this group only 2.3 per 

 cent of the surviving embryos were double specimens, but almost 20 per 

 cent of those actually deformed were of this type. 



When 25 days old, man}^ of the normal specimens in both the Ai 

 and A2 groups had hatched, although all of these were far behind the 

 control, which had begun hatching when 12 days old. 



The actual percentage of double individuals induced by this 

 experiment is not really large, yet it is comparatively very sig- 

 nificant. From a long experience with these eggs I would ven- 

 ture to believe that under normal developmental conditions there 

 is only a small chance for finding one double specimen among a 

 thousand. During the past three spawning seasons a great num- 

 ber, certainly many thousand, of Fundulus eggs have been ar- 

 rested in their development by being placed in low temperatures 

 after the germ-ring had begun to form. These specimens were all 

 examined with such care in connection with the various problems 

 being studied that no double specimen could have escaped record. 

 Yet among all these late arrests not one double individual 

 existed. 



