276 SALMONID^ OF BRITAIN. 



nerve with a double eyeball, or the nerves may be present but the eye absent. 

 How these additional eyes have been developed has been a cause of dispute.* 



The types of simple monsters may generally be classed under one cause, arrest 

 of developmentf occurring in the normal course of embryonic life. And these 

 may be shown in the head wherein the eyes, mouth, upper jaw, lower jaw, or 

 opercles may be affected : or the body as in some portion of the vertebral column, 

 or in the fins which may be shortened, lengthened, or the rays in an abnormal 

 condition. 



Sometimes the upper jaw is the shorter, sometimes the lower, as may be seen 

 figured in plate xii, and these monstrosities, especially the latter, are common 

 among hybrids and fish raised from young parents. Although these monstrosities 

 may be occasioned by increased growth, they are more commonly due to the arrest 

 of development in some of the bones of the head. 



The bull-dog deformity of the snout or an arrest of development in the 

 premaxillaries (plate xii, fig. 19) is by no means rare, more especially in fishes 

 raised by the fish-culturist. The example figured was sent me from a burn near 

 Perth, but I have also examples of fontinalis from Howietoun with a similar 

 deformity, one of which was a female, from it many eggs were obtained, but none 

 of the young were thus malformed, it not being inherited. J 



There may be an apparently shortened lower jaw, as is more commonly seen 

 in hybrids or in fish artificially raised, than in those in their natural condition. 

 Many, however, do not seem to be bom thus, and at Howietoun it has been 

 observed to be most common in such as are kept in wooden tanks, and supposed 

 to be owing to their using their lower jaws with injurious force against the 



* M. Camille Dareste (Arch. Zool. Exp^. et Gen. v, 1876) entered very fully into this question, 

 least, among the higher vertebrates. The type of monsters, as he observed, which appear first in 

 the embryonic evolution is that characterized by an arrest of development in the head, which shows 

 neither eyes, nose, or buccal apparatus. This is rather rare. The head consists of a single bud, 

 presenting in its lower part a cul-de-sac, the pharynx. He had observed, in fact, a great number 

 of times, very diverse anomalies of the primitive groove in the region of the head. In many of 

 these he saw that the primitive groove had not attained to the anterior extremity of the head. 

 It is evident that under such conditions the anterior cerebral vesicle and the ocular vesicle 

 which are dependent on it, cannot be formed ; or else that the vesicle is incompletely formed and 

 becomes constituted as a simple rudiment. If the embryo continues to develop it presents the 

 fundamental character of an undeveloped head. 



The next type, Cyclops, or a single eye in the median line of the face, or formed of two 

 conjoined into one, or two eyes in one orbital space, or even in two orbits placed very close 

 together or nearer than in a natural state, are merely degrees of one type. Geoffroy Saint- 

 Hilaire, relying solely on the study of these monsters themselves, gave an account of cyclops 

 formed by atrophy of the nasal apparatus and a more or less complete fusion of the eyes. Husche, 

 on the contrary, began by observations of facts in embryology, and explained aU these forms of 

 Cyclops by an arrest of development. He believed that he had observed the ocular apparatus was 

 single in its origin, and formed a vesicle situated at the extremity of the cerebro-spinal tube 

 immediately in front of the first cerebral vesicle. This single ocular vesicle enlarged in size 

 transversely, and then divided into two parts and finished by constituting two distinct ocular 

 vesicles, and situated on the two sides of the head. The interspace between the two ocular 

 vesicles became occupied, little by little, by a prolongation of the cerebro-spinal tube, which first 

 formed the vesicle of the third ventricle, afterwards the vesicle of the cerebral hemisphere. 



M. Dareste, however, considered that the ocular vesicles are separated from their first 

 appearance, and that the cyclops is not the persistence of certain embryonic conditions but an 

 arrest of development. 



t One very general cause of malformation is attributed to impeded circulation in the foetus, 

 possibly first through the circulation, and subsequently through the nervous system. Vrolik 

 remarked that " I presume to conclude that no malformation whatever proceeds from a. central 

 system, but is occasioned merely by impeded development, the cause of which remains concealed. 

 This impediment may be confined to one part, or may be extended over more." Abnormal 

 development gives monsters wherein there is a deficiency if impeded, and excessive formation 

 when such is in excess. 



J Mr. Eagle-Clarke sent me, 13th November, 1885, a specimen of trout from Penyghent {see 

 Yorkshire Vertebrata, p. 127), with the under jaw projecting beyond the upper. All the fish in 

 the beck (which is on the mountain side and has only a course of a short half mile, when it 

 disappears into a deep abyss ih the limestone), are of the same variety, and locally termed 

 " ground trout." 



The late Mr. Arthur writing from Dunedin, April 2nd, 1886, remarked of Lochleven trout 

 which arrived in New Zealand " a year ago, a Government lot, have developed a singular deformity 

 in the shortening of the lower jaw." 



