132 H. H. NEWMAN. 



temperatures, or hybridization, the result is always the 

 same: some individuals are affected in one way, some 

 in another, and some not all. A lot of Fundulus eggs, for 

 example, are exposed for a given time to a given intensity of 

 X-rays. Some of the radiated individuals, as Miss Hinrichs has 

 shown, develop into two-headed monsters, others become cy- 

 clopic, others show lesions of the heart or circulation, not to 

 mention many other typical teratological conditions. Our pre- 

 sent problem does not concern itself with any attempt to account 

 for the individual differences in susceptibility among eggs of the 

 same batch : this is a problem of heredity, not one of development. 



To revert specifically to the present experiments, it should now 

 be said that, although our attention has been focused upon 

 symmetry reversal to the exclusion of any other effects of exposure 

 to low temperatures, it would be unfortunate to leave the reader 

 with the impression that everything else about the experimental 

 larvae was strictly normal. The truth is that some larvae develop 

 no ccelomic structures at all ; that others show a tendency toward 

 a metameric duplication of anterior or posterior coeloms; that 

 occasionally a larva has a hydropore connected with the posterior 

 ccelom as well as the anterior. It should also be added that in all 

 experiments in which the icing of larvae lasted more than about 

 two hours, many other expressions of growth inhibition appeared, 

 sometimes associated with symmetry reversal and sometimes not. 

 My strong impression was that on the whole the larvae with 

 reversed asymmetry were rather more normal in other respects 

 than those with typical asymmetry. 



In conclusion then, it may be said that symmetry reversal, and 

 that particular phase of it which we may call induced bilaterality, 

 are merely some of the specific effects resulting from arresting the 

 growth of the larvae at the blastula stage. If the arrest takes 

 place much earlier or much later than the blastula stage other 

 expressions of differential inhibition prevail and there is no evi- 

 dence of symmetry reversal. If eggs are iced during early 

 cleavage or even before cleavage the particular response is 

 twinning, while icing at other times give equally striking but 

 different results. These matters, however, must be left for 

 another study and another report. 



