254 PATTERNS AND PROBLEMS OF DEVELOPMENT 



if not too much inhibited, may become young ascidians before hatching. 

 Developmental stages after resorption of the tail are much less susceptible 

 than those of larval development; and differentially inhibited larvae in 

 which a tail never develops may become apparently completely normal 

 ascidians, though many of them fail to develop beyond early stages of 

 metamorphosis. Much inhibited forms sometimes hatch but are incapable 

 of movement, or the aborted tail may show only slight muscular tremors 

 (Fig. IOC, F,G). 



VERTEBRATES 



Extended consideration of the enormous literature of vertebrate tera- 

 togeny and teratology is quite beyond the present purpose. We are pri- 

 marily concerned here with developmental modifications, apparently dif- 

 ferential in character, that is, not specific for particular agents, and re- 

 sulting from controlled exposure of the entire organism in early stages 

 of development, or of egg or spermatozoon, to the action of physical and 

 chemical agents or conditions outside the range of so-called "normal en- 

 vironment." These experimental modifications throw some light on the 

 problem of origin of certain of the "accidental" teratological forms result- 

 ing from uncontrolled and unknown conditions. One of the most interest- 

 ing results in this field of experimental vertebrate teratogeny is the high 

 degree of similarity of teratological forms produced by different external 

 factors. Although various authors have, from time to time, regarded 

 terata produced by certain agents as specific for these agents, further ex- 

 periment has shown that the supposed specificity did not exist. In the 

 more advanced developmental stages after a considerable degree of dif- 

 ferentiation particular agents may act more or less specifically on par- 

 ticular organs; but in the earlier stages less, or no, evidence of such local- 

 ized specific action appears. If the yolk content differs greatly in different 

 regions of the egg or embryo, as in amphibians, the yolk-laden parts may 

 be more susceptible to certain agents and less susceptible to others than 

 those with little yolk. This, of course, represents a specific difference re- 

 sulting from regional differentiation already present in the egg. However, 

 aside from such differences as these, the general similarity of the modifica- 

 tions produced by many different agents is evident, even in the work of 

 the earlier investigators in this field, ^ and some of them called attention 

 to it. The evidence from later work supports the view that the modifica- 

 tions of early stages of vertebrate development by external agents depend 



3 See, e.g., Dareste, 1891; O. Hertwig, 1892, 1895, 1898; Gurwitsch, 1895; Bataillon, 1901, 

 1904; Rabaud, 1901-2. 



