PRODUCTION OF MONSTERS BY HYBRIDIZATION. 315 



(A) FORMS RESULTING FROM DIFFERENTIAL INHIBITION. 



In fish monsters, whether they are the result of chemical, 

 physical, or biological (hybridization) inhibitors, the most preva- 

 lent abnormalities are associated with the eyes and with the 

 heart. The teratomata resulting from all three types of in- 

 hibiting agents are strikingly similar, so I shall from now on 

 confine my description and discussion to monsters produced by 

 heterogenic hybridization. In any heterogenic cross the least 

 abnormal forms are always those that have something slightly 

 abnormal or sub-normal about the eyes or the heart. We 

 must conclude then that the primordia of these two structures 

 are peculiarly susceptible to agents that retard metabolic rate. 



OPHTHALMIC TERATA. Eyes in the least extreme cases may 

 be either a little too small, a little too close together, of unequal 

 size, or asymmetrical in position. In the most extreme cases 

 the eyes may be wanting, or very poorly differentiated ; the eyes 

 may be fused more or less completely (various grades of cyclopia) 

 or there may be a single unilateral eye. Every intergrade 

 between the least extreme and the most extreme ophthalmic 

 terata may be readily found in almost any hybrid experiment. 

 Why in a single batch of eggs there should be so highly diversified 

 a .result is not known, but it must be due to physiological (age, 

 maturity, etc.) differences in the individual eggs or sperm, or 

 a combination of both. 



In general we may conclude that suppressed eyes, either bi- 

 lateral or unilateral, are the result of inhibition primarily of the 

 apical end of the apico-basal gradient, while eyes too close 

 together or fused in the median line (cyclopic terata) are the 

 result primarily of inhibition of the median portion of the 

 mesio-lateral axis. 



CARDIAC TERATA. All grades of heart abnormalities occur in 

 heterogenic crosses. The less extreme cases are those in which 

 the heart development is belated relatively to the other bodily 

 regions, or in which there may be a failure of the whole cardiac 

 mechanism to become inclosed within the body cavity. In such 

 cases the heart remains outside the body and there is usually 

 a much enlarged pericardium. In the most extreme cases the 

 heart never develops, or becomes a mere pulsating strip of muscu- 



