160 VERTEBRATE ZOOLOGY 



suppressed. Some of the most extrem.e examples of this condition 

 have just been called to our notice in the description of the last two 

 sub-orders dealt with, the Pediculati and the Plectognathi. These 

 types of structural distortion may be called for convenience head-fish 

 (megacephalic) types. Two other types not directly related to the 

 two primary types just mentioned, but more or less closely correlated 

 with them, are: first, the short, high, compressed type; and the second, 

 the low, laterally expanded type. Both of these types agree in having 

 the primary axis foreshortened and the subordinate axes exaggerated. 

 The first involves an exaggeration of the dorso-ventral (secondary) 

 axis at the expense of the primary and tertiary axes. In extreme 

 cases the dorso-ventral diameter exceeds the length; and the dorsal 

 and ventral integumentary elements, such as fins and spines, become 

 greatly lengthened and specialized, as in Zanclus (Fig. 86). The sec- 

 ond type shows a dominance of the tertiary axis (bilateral) over both 

 primary and secondary axes; and the result is a very wide type, with 

 expanded and specialized pectoral fins, the pelvic fins having been 

 relatively suppressed in the foreshortening process that has affected 

 the primary axis. 



All of these specialized and degraded types of fishes may be reduced 

 to two categories: — a, those in which there has been a relative sup- 

 pression of the apical parts of the various axes, especially the primary 

 axis, accompanied by a relative emancipation of the subordinate axes 

 from the dominance or control of the primary axis; b, those in which 

 the apical parts of the various axes have become relatively highly 

 specialized or exaggerated, while the basal elements have become 

 relatively suppressed. 



In the opinion of. the writer, all of these conditions can be readily 

 interpreted as the morphological consequences of growth-inhibiting 

 agents, acting during the ontogeny of the individuals. The morpho- 

 logical equivalents of all of these exaggerated types of natural fishes 

 can be experimentally simulated by the use of inhibiting agents 

 applied to the eggs or young embryos of generalized species of fishes. 

 The writer and other investigators have performed extensive series of 

 experiments with the eggs of Fundulus (Fig. 79) and other generalized 

 types of teleosts, using a wide variety of growth-depressing agents, 

 such as anaesthetics, low temperatures, heterogenic hybridization, 

 etc. If, for example, the eggs or early embryos are placed for limited 

 periods in weak sea-water solutions of alcohol or potassium cyanide, 



