HYBRIDS BETWEEN FUNDULUS AND MACKEREL 411 



effect upon the development. At the same time the specificity 

 of the paternal chromatin seems to be neutralized and it is 

 unable to cope with the unaltered maternal hereditary factors 

 in the determination of chromatophore and other characters. 

 Other hybrid eggs react a little less promptly or less completely 

 to the foreign materials and are in consequence affected by them 

 more or less strongly. Any agent inimical to development seems 

 to express itself primarily as a retarding agent. As Child has 

 shown, an inhibiting or retarding agent has the most pronounced 

 effects upon the structures that have the highest rate of metabo- 

 lism, usually the head, heart, and especially the eyes. Conse- 

 quently, when the foreign materials are most active, as shown by 

 the presence of many of the paternal type of chromatophores, 

 the effects of retardation in development are most strikingly 

 shown in pronounced abnormalities or early death and disinte- 

 gration of embryos. In the average case, however, the inhibiting 

 effect, though long continued, is not UAduly severe, and there 

 result large numbers of embryos with abnormal head parts of all 

 grades of severity and fairly well-developed posterior structures. 

 This type of result would be classed as the effects of differential 

 inhibition. 



There occur, however, not infrequently embryos in which the 

 head parts are better developed than are the posterior parts, or, 

 in extreme cases, only isolated eyes or hearts occur on an other- 

 wise undifferentiated blastoderm. These are, I believe, to be 

 interpreted as the result of differential inhibition followed by 

 differential recovery. According to Child's theories, the apical or 

 head parts, although most susceptible to retarding conditions, 

 also have the greatest capacity for i-ecovery when the inhibiting 

 agent is removed or reduced in intensity. Evidently some 

 embryos recover from the inimical effects of the foreign sperm, 

 either by becoming acclimated or because the foreign element has 

 grown weak, or perhaps been absorbed, and the anterior struc- 

 tures take up the process of differentiation and develop principally 

 eyes and adjacent head parts, the rest of the body remaining 

 in the original retarded condition incapable of recovery. Thus 

 we can account for trunkless heads and even isolated eyes and 



