REDUCTION AND PARASITISM 59 



one there can be little doubt. I believe, on general grounds, that it will also be found capable 

 of being produced by the action of external factors, but meantime direct experimental evidence on 

 the question is awanting. M'Intosh (155) has noted two points of great interest, (a) that in Pleuro- 

 nectids which are metamorphosing normally, the future under side is paler than the other, even 

 before the young fishes begin to swim obliquely or to leave their pelagic habit of life ; (&) that in 

 particular instances where the pelagic life was unduly prolonged, the same lessening of colour 

 occurred. 



REDUCTION AND PARASITISM. 



This condition occurs in each of the classes of double monsters. All degrees of reduction on 

 the part of the minor twin may have been attained even before the end of embryonic life. In the 

 adult, supernumerary paired fins (p. 54) are probably the structures which most readily give evidence 

 of an originally double condition, obscured later by almost complete degeneration of one of the 

 components. Lereboullet's work provides cases in which the process of reduction of a twin rudiment 

 was actually observed in the embryo. In the trout, out of many examples, I shall only select two 

 for particular description, and both of them belong to Class I. (p. 12). 



The first specimen is in some respects unique. My attention was directed to it by the 

 presence of an interruption or cleft in the right upper jaw, producing the appearance of a right- 

 sided harelip in what seemed to be, in other respects, a normal newly-hatched trout embryo. 

 On cutting serial sections, I found that a small additional eye lies at the bottom of this cleft, 

 in the roof of the mouth, to the right side of the middle line, and in the same transverse plane 

 as the normal eyes. This eye is embedded in confused muscular tissue, has a well-developed 

 lens, a small retina, no choroidal fissure and no choroidal gland. Its optic nerve is represented 

 by a small bundle of fibres which sweep over the edge of the retina to join the right normal 

 optic nerve (PI. XXIV. fig. 105). The retina is small and elongated antero-posteriorly. The 

 pigment-layer is present as such only in the posterior half of the retina. Anteriorly, the corresponding 

 layer is non-pigmented, richly cellular, and becomes continuous with the brain-wall just in front of 

 the optic recess, in such a way that the central cavity of the brain is prolonged into the space 

 between the retina and the pigment-layer. An optic stalk, embryonic in condition, is thus present. 



Two deep grooves are found in the floor of the third ventricle and the mid-brain, each leading 

 down into a separate infuudibulum and hypophysis. The grooves are separated by a considerable 

 ridge of brain-tissue. The right hypophysis and its hypoaria are somewhat compressed ; the rest of 

 the brain is normal. The right palato-quadrate bar is absent and the trabeculae cranii are displaced 

 to the left. 



Taken by itself, the supernumerary eye might seem to be simply a case of repetition, since its 

 nerve is derived from the right optic nerve. But the persistence of an embryonic optic stalk, 

 together with the presence of a double hypophysis in the brain, would seem to indicate that the 

 explanation is to be found in an extremely local degree of axial duplicity. It is not clear, 

 however, whether the accessory parts are to be looked upon as being the remains of inner 

 (adjacent) structures in a double-head, or as representing a reduced second head. A somewhat 

 analogous case is described by Gurlt (Lekrbuck der pathologischen Anatomic, ii. Theil, p. 221, 

 Berlin 1832). In this instance the right ramus of the lower jaw in a lamb has an accessory ramus 

 on its inner side with an accessory set of molar teeth. The tongue is double anteriorly. There 

 are two pituitary glands and two infundibula arising from a single large tuber cinereum, two 

 pineal glands, three pairs of corpora quadrigemina, and two aqueducts of Sylvius. Three accessory 

 nerves, arising from the mid-ventral line of the brain, go to an " ocular rudiment in the sphenoid." 

 This account is quoted from Taruffi (Storia della Teratologia, vol. iii. p. 115). For other examples 

 of duplicity of the hypophysis see Ahlfeld (Die Missbttdungen der Menschen, p. 73, Leipzig 1880) 

 and Bland Sutton (Transactions of the Odontological Society (Dental Record}, 1888, pp. 73-78). 



In the trout, if new and more definite cases of the kind described above become available 

 to found on, it may be necessary to institute a Class of anadidymi which will come in front of 



