450 DU. J. F. GEMMILL OX [^^W- 1*^) 



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. 



Tioo deep grooves are found in the floor of the third ventricle 

 and the mid-brain, each leading down into a separate infundibulum 

 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 trabeculse cranii are 

 displaced to the left (fig. 1, PI. XXXIII.). 



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, indicates 

 rather that the explanation is to be found in an extremely local 

 degree of axial duplicit)* which has become obscured by the growth 

 of the predominant twin head. A somewhat analogous case is 

 described by Gurlt (' Lehrbuch der pa-thologischen Anatomie,' ii. 

 Theil, p. 221 : Berlin, 1832). 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 Silvius. 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 

 Tarufii (' Storia della Teratologia,' vol. iii. p. 155). For other 

 examples of duplicity of the hypophysis see Ahlfeld (' Die Miss- 

 bildungender Menschen,' p. 73 : Leipzig, 1880) and Bland Sutton 

 ('Transactions of the Odontological Society,' 1888). 



The second specimen was quite normal in appearance, except 

 for the presence of a tiny refractive knob behind the left eye. 

 Examination of serial sections showed the knob to contain a lens 

 of considerable size, enveloped in muscle-fibre, but unaccompanied 

 by any other eye-structure, lying in front of an exceedingly 

 minute fore-brain and third ventricle. The cavity of this third 

 ventricle comnumicates with the mid-brain cavity of the normal 

 head (Plate XXXTII. fig. 2). The embryo was quite lively when 

 obtained, and its chances of survival would probably not have been 

 appreciably diminished by the small tumour in question. It will 

 be seen fi'om fig. 2 that the functional eyes and fore-brain belong 

 to a predominant 7Hght twin head, as also do the olfactory organs, 

 the mouth, and the anterior cranial cartilages generally. The 

 back part of the brain and the whole of the body are, however, 

 composite, since the left moiety of them represents structures 

 which are continuous with the left side of the left (aborted) 

 twin, while their right side is a continuation backwards of the 



