14 DOUBLE MONSTROSITY STRUCTURE, CLASS I. 



and left twin heads. The succeeding cranial nerves are also normal, in the sense that there is only 

 a single pair of each. A rudiment of inner or adjacent pairs of trigeminal ganglia may be recognised 

 in the form of a thin elongated band of tissue containing small nerve-cells and lying in the middle 

 line underneath the region of the pons. This band of tissue has no central or peripheral nerve-fibres. 

 The pons and cerebellum are single, but their internal structure shows traces of duplicity, especially 

 in the case of the pons. The medulla oblongata is slightly expanded transversely, but is otherwise 

 normal in form. 



Spinal Cord. In the anterior part of the spinal cord there is a curious and interesting 

 reappearance of duplicity, coextensive with the duplicity of the notochords, and with the presence, 

 ventral to them, of a median composite muscular mass representing united adjacent lateral muscles. 

 In this region, the spinal cord is greatly expanded in a transverse direction ; its cavity is spindle- 

 shaped, and, in addition to the usual nerve-roots, it gives off, on the ventral aspect in each segment, 

 a pair of small additional motor roots which are distributed to the median muscular mass just 

 mentioned. 



Organs of Sense. There are two pairs of olfactory organs and nerves. The outer eyes (right 

 eye of right twin head and left of left head) are normal, but lie further back than usual, so that 

 their optic nerves pass backwards as well as outwards from brain to eyeball. The inner or 

 adjacent eyes may, or may not, be fused with one another. In the former case (which is also 

 much the commoner) there is a single lens, which is sometimes smaller than normal, but sometimes 

 larger and showing evidence of duplicity ; the sclerotic and choroid coats are single ; the retinae 

 never fully unite, each showing its own choroidal fissure, optic nerve, and choroidal gland. In all 

 cases the external recti muscles belonging to the inner eyes are absent ; the superior obliques are 

 absent or rudimentary, but the remaining ocular muscles are present in two sets. 



Heart and Vessels. The heart and the ventral aorta are normal, but the dorsal aorta and its 

 roots, as well as the choroidal and carotid arteries, require description. The coming together of the 

 main collecting-trunks on either side to form the dorsal aorta is carried backwards for a very con- 

 siderable distance, and takes place only at the level of union of the notochords. The pseudobranch 

 on either side receives a branch from the hyoid artery, and its efferent vessel passes to the choroidal 

 gland of the corresponding (i.e. outer) eye. The choroidal glands of the inner or adjacent eyes are 

 supplied by blood which has not passed through the pseudobranch. In the specimen from which 

 PI. VIII. fig. 35 is taken, the arrangement of vessels is quite symmetrical and is indicated in 

 PL XVIII. fig. 67. A transverse arch vessel connects the upper aortic roots, and gives off a common 

 choroidal artery which soon bifurcates. The two resulting vessels pass through the separate 

 pituitary spaces, and are distributed to the choroidal glands of the adjacent eyes. 



As is well known through the work mainly of Johannes Muller, 1 the pseudobranch and the 

 choroidal gland are so related to one another in osseous fishes that a pseudobranch is never present 

 in species which have no choroidal gland, while in rare instances only is a choroidal gland present 

 where there is no pseudobranch. It is therefore of considerable interest to note that the choroidal 

 glands of the inner or adjacent eyes, in the type of monstrosity under consideration, derive their 

 blood-supply directly from the first aortic root. There is a certain amount of variation in the exact 

 mode of origin of the choroidal and carotid arteries. For instance, cases occurred in which these 

 vessels all arose from the first aortic root on one side only, instead of both roots participating equally 

 as in the specimen figured (PI. XVIII. fig. 67). 



Kidneys, etc. The mesonephros, ureters, bladder, and urinary pore are normal, but the 

 pronephric glomerulus is composite, or may be double. An example of the composite condition is 

 figured in PI. XIX. fig. 84. The glomerulus is larger than normal, and contains two vascular tufts 

 between which is a median compartment that obviously corresponds to the fused adjacent halves of 

 a pair of glomeruli, but has no Wolffian ducts in connection with it. In the specimen from which 

 Fig. 85 is taken the two glomeruli are separate, and one of them has a rudimentary Wolffian 

 duct arising from its inner side, while the other has none at all. The two figures should be com- 

 1 Vergleichende Anatomie der Myxinoiden, Berlin 1835-1845; 3 te Fortaetz. 1841 (41-99). 



