MINOR ABNORMALITIES— HUMP-BACK, ETC. 53 



(c) Some of the sharpest twistings are exhibited by hemididymous or mesodidymous forms, after 

 exhaustion of the yolk store;. 



(/) Irregular curvature may be associated with absence, more or less complete, of the great 

 lateral muscle masses on one side. In a crooked specimen examined by the method of serial sections, 

 I found that a short distance in front of the vent the muscle masses vanish quickly and completely 

 on the left side (Text-fig. G). 



The blank thus left is filled up in small part by loose connective tissue, but is chiefly 

 compensated for by the mid-dorsal and mid-ventral structures curving round so as to encroach on 

 tin? left side. In this manner the dorsal edge membrane and the left pelvic fin approach each other 

 until they are separated by less than a fourth part of the circumference of the body. All the other 

 organs tend to be displaced or modified in form so as to suit the new shape. A transverse section 

 in front of the vent is illustrated in the accompanying figure. Just behind the vent the proper 

 muscle tissue of the left side reappears, but in an exceedingly incomplete manner, and this condition 

 persists to the end of the tail, which never fully recovers its bilateral symmetry. The tail fin is 

 bunched together so that at first sight it appears to be absent. I would refer the origin of this 

 deformity to an early katadidymous condition, in which, later, the left component became completely 

 lost. 



Causation. We are still in the dark as regards the general question of the causation of 

 "twisted bodies" and the allied deformations. No doubt they are mainly congenital, and it is probable 

 that their production depends oil some very early developmental aberration capable of being induced 

 by the action of external factors. But I believe the corollary is that (as in pug-head, cyclopia, etc.), 

 certain forms of this defect are likely to appear from time to time spontaneously, by abrupt 

 autogenetic variation. In the latter case they may be transmitted to descendants provided they 

 permit growth and survival into adult life. Thus the goldfish races referred to by Tornier (24-9) 

 indicate that in some of its forms the condition is hereditary, and the same thing is very strongly 

 suggested by various facts relating to the widely distributed and constantly recurring Lord-fish 

 type of cod, although Day and others class this type among " diseased " forms. 



Mechanical injury has naturally been thought of as a possible cause. Thus Day (5 '4) suggests 

 that the hump-backed condition of many trout in certain hill streams might be due to violence 

 through rolling over small cascades and falls at an early stage. Experimentally it is stated that 

 twisted body is one of the defects that tend to appear if developing eggs are subjected to mechanical 

 disturbance, or to unfavourable influences of other kinds. Holt (97) quotes Kyder as giving 

 some data on the influence of cold on its production, while Wdliamson (269a) refers to similar data 

 noted by Meyer for the herring. 



In certain twisted trout embryos examined by the method of serial sections I noted the 

 presence of premature calcifications within the sheath of the notoehord. Conceivably, localised 

 conditions of this kind might, by preventing normal axial elongation, lead in some cases to curvature 

 of the backbone and in others to shortening without curvature. The membranous character of the 

 ossification in the formation of vertebral centra seems highly favourable to coalescence of centrum- 

 units wherever very marked antero-posterior shortening occurs, especially if the shortening be 

 accompanied by tissue irritation such as pressure might produce. Something of the kind is 

 perhaps indicated by the fact (Howes 102a) that the weight of the shortened coalesced centra may 

 be greater than that of an equal number of normal ones. 



It has been suggested (Bugnion 34, Howes 102a) that the body muscles may be primarily at 

 fault, through undue rigidity, contraction, or failure in growth. There is no convincing evidence 

 for this view, however, except in the type described under (/) above, and here the original defect 

 must date as far back as the period of actual formation of the muscle plates. 



It need hardly be added that the condition is likely to be due sometimes to ordinary 

 pathological conditions, for example, to inflammation, or to some other form of bone disease. 



