DIFFERENTIAL DEVELOPMENTAL MODIFICATION. Ill 267 



radiated ova fertilized by normal spermatozoa, have been found to give 

 developmental modifications essentially similar to those resulting from 

 direct action of inhibiting agents on developmental stages; treatment of 

 ova or spermatozoa with chemical agents has given similar results.'" In 

 the case of injured spermatozoa the effect of the injury on development 

 is, in general, greatest in the regions most susceptible to inhibition by 

 direct action of external agents, the regions of greatest developmental ac- 

 tivity; that is, the cells of these regions with injured nuclei do not attain 

 the physiological levels essential to full development, but development 

 at lower gradient levels is less, or not at all, affected. 



It has been shown that certain heterogenic hybrids, notably among the 

 fishes, often show inhibitions of development similar in character to the 

 differential inhibitions by external agents. In a series of papers concerned 

 with fish hybrids particular attention has been called to this point by 

 Newman." He has noted in some detail the similarities of the hybrid 

 terata to those determined by direct action of chemical and physical 

 agents. In these hybrids development is retarded, and the monsters usu- 

 ally show what appear to be various degrees of differential inhibition, the 

 heads being small, eyes approximated or cyclopic, the heart, and in some 

 the posterior part of the body, inhibited as with external agents. More- 

 over, forms similar to cases of differential tolerance, conditioning, and 

 recovery also appear. The similarity of hybrid monsters to modifications 

 produced by external inhibiting agents was also noted by Loeb (191 2, 

 191 5). More recently similar terata have been reported by Montalenti 

 (1933) as characteristic of the amphibian cross Bujo viridis 9 X B. vul- 

 garis & but infrequent in the reciprocal cross. 



Various suggestions have been advanced in attempting to account for 

 the factors concerned in these modifications of hybrid development: toxic 

 action of the sperm, in general more severe the less closely related the 

 species crossed; nuclear ''incompatibility" resulting in aberrant distribu- 

 tion of chromosomes or perhaps in inactivation or loss of chromosomes of 

 parts of chromosomes; alteration of the nucleoplasmic ratio; asynchrony 

 of developmental rate, particularly in the case of sperm from a species 

 with slower development than the maternal species. 



" See, e.g., Opperman, 1913, fish spermatozoa, radium; Gee, 1916, fisli spermatozoa and 

 ova, alcoiiol, NaOH; Bardeen, 1907, amphibian spermatozoa. X-rays; O. Hertwig, 191 1, 1913, 

 amphibian spermatozoa, radium; G. Hertwig, 191 1, amphibian ova, radium; G. and P. 

 Hertwig, 1913, amphibian spermatozoa, nicotine, strychnine, chloral hydrate, various dyes; 

 Stockard, 191 2, 1913, guinea-pig spermatozoa and ova, alcohol. 



" Newman, 1908, 1914, 19150, 1917&, 1918. 



