266 PATTERNS AND PROBLEMS OF DEVELOPMENT 



organs, their absence, anencephaly, and acephaly all occur in the chick. 

 The developing heart is, in its earlier stages, a region of high susceptibility 

 and frequent, apparently differential inhibition. Decreased elongation, 

 spina bifida of various degrees at various levels, and inhibition of posterior 

 regions are, as Hyman points out, associated with the high susceptibility 

 of the region of posterior elongation which becomes separated from the 

 highly susceptible head regions with the progress posteriorly of the node 

 and primitive streak. With sufficient inhibition the whole embryo may 

 be absent.^ 



The early developmental stages of mammals are, as yet, scarcely ac- 

 cessible to long-continued experiment with altered environment. The ef- 

 fects on the nervous system of irradiation with X-rays and radium (Bagg, 

 1922) and of administration of alcohol to parents suggest embryonic dif- 

 ferential susceptibility. In some of these experiments, however, the direct 

 effect of the experimental conditions is on the germ cell rather than on 

 the embryo; in all cases further experiment is necessary with data con- 

 cerning early stages. 



OTHER FACTORS IN MODIFICATION OF VERTEBRATE DEVELOPMENT 



In his experiments on development of ligatured Triton eggs Spemann 

 (1904) found that, when the ligature was slightly oblique to the median 

 plane, more or less complete duplication of the anterior end resulted, as 

 with median ligatures, but that the head developing on the side with 

 less of the median anterior region may show all gradations of inhibition 

 to complete cyclopia, the modifications being the same as the differential 

 inhibitions produced by chemical and physical agents. In the light of 

 more recent work of Spemann and his school it seems evident that the 

 inhibition of head development is associated with inadequacy of induc- 

 tion, perhaps also with absence of the apical ectoderm. 



Inhibition of development, apparently differential in character, follow- 

 ing injury to the spermatozoon, is of particular interest because the in- 

 jury, doubtless chiefly of the sperm nucleus, appears in development as 

 inhibition with regional differential along one or more axes, as in cases of 

 direct exposure of the ovum or developing embryo to an inhibiting agent. 

 Eggs fertilized by spermatozoa irradiated by X-rays or radium, or ir- 



9 For references to, and discussion of, the work of earlier experimenters — Fere, Windle, 

 Kaestner, Mitrophanow, and others — see Hyman, 1926a. See also the following: Rabaud, 

 1901-2; Oilman and Baetjer, 1904; Tur, 1904; Reese, 1912; Stockard, 1914; Alsop, 1919; 

 Riddle, 1923; Byerly, 1926; Buchanan, 1926c; Hinrichs, 1927; Gradzinski, 1933, 1934; also 

 citations by these authors. 



