DOUBLE MONSTROSITY STRUCTURE, CLASS I. 13 



Cranial Skeleton. There are two notochords in front, and therefore, potentially at least, two 

 pairs of parachordal cartilages, but the four cartilages are united to form a single basilar plate. In 

 front, the two nasal cartilages are placed widely apart ; each contains a right and left olfactory pit, 

 and is continuous behind with its own trabeculae cranii. The two pairs of trabeculae converge as they 

 pass backwards ; their inner 1 or adjacent elements unite to form a median flattened bar, which joins 

 the basilar plate mentioned above as being formed by the parachordals. At the same time, the 

 outer elements of each pair of trabeculae have diverged from the inner elements to enclose a pair of 

 pituitary spaces, the latter lying one on each side of the median bar formed by union of the inner 

 trabecular elements. There are only two auditory organs, and their cartilages are continuous with 

 the outer trabecular and parachordal elements in the floor of the skull. Dorsally, the auditory 

 capsules are connected over the cerebellum by a vault of cartilage, which is narrower antero- 

 posteriorly than in the normal condition. Over the medulla the laminae of the parachordals 

 nowhere form a complete vault. Accordingly, the posterior fontanelle is much larger than in a 

 normal case. Dorsally, each nasal capsule is continuous with a pair of supraorbital bars, of which 

 the outer elements pass backwards on either side to join the auditory capsules, while the inner or 

 adjacent elements are connected with each other and with the outer bars over the pineal body. 

 Behind this the adjacent bars disappear, the result being that over the region of the cerebral lobes 

 there are two small anterior fontanelles, while over the optic lobes there is a single large fontanelle. 



Visceral Arch Skeleton. The inner or adjacent palato-quadrate bars converge, fuse, and end 

 abruptly without being attached to a suspensorium ; the outer bars are continued backwards on 

 either side, and articulate with the hyomandibulars attached to the auditory capsules. Articulating 

 with the united part of the inner palato-quadrate bars is a small twisted piece of cartilage, which 

 passes downwards in the septum between the two mouth-openings, and represents an inner or 

 adjacent pair of Meckelian cartilages. The corresponding outer Meckelian cartilages meet in the 

 septum between the two mouth-openings, but do not form a symphysis. They are continued back- 

 wards to articulate each with a normal suspensorium attached to the outer side of the corresponding 

 periotic capsule. The hyoid bars and the branchial cartilages are normal, except that they diverge 

 rather more widely than is usual. Each arch, however, may be looked upon as being composite, 

 namely, as consisting of the outer components of a double set of arches, the inner components 

 having been lost. 



Notochords, etc. Two notochords are present as far back as from the 14th to the 20th 

 body-segment. These converge at an acute angle and finally unite. The disposition of the 

 neural and haemal arch cartilages is as follows. Where the two notochords are some little distance 

 apart, the inner or adjacent neural arches are displaced so as to form a floor for the transversely- 

 expanded spinal cord, while the inner haemal arches lose their ventral processes and become smaller. 

 As the notochords come closer together and unite, the inner neural and haemal arches disappear, 

 while the outer arches gradually assume a normal form and position. 



Brain and Cranial Nerves. There are two pairs of cerebral lobes and two thalamencephala, 

 diverging forwards from a single composite optic lobe region. The cerebral lobes and thalamen- 

 cephala, besides diverging, are rotated slightly in such a way that they lie closer together dorsally 

 than ventrally. There are two pairs of olfactory nerves, two pairs of optic nerves, and two 

 pineal diverticula. There are also two infundibula, which converge as they pass downwards and 

 backwards. Each ends in a hypophysis sac after giving off the usual diverticula for the hypoaria. 

 The inner hypoarium on either side, owing to want of space, is smaller than normal, and lies above 

 and in front of the outer hypoarium. The optic lobes show a remarkable transition between the 

 double and the single condition. Their cavity and roof-parts are single, while the basal structures 

 are doubled. There are thus two pairs of 3rd nerves. But only a single pair of trigeminals is 

 found, the components of which represent, respectively, the right and left 5th nerves of the right 



"In examples of lateral union like the present one, the terms inner or adjacent refer to those structures which 

 gradually become approximated as the twins converge. Here, accordingly, the left trabecula of the right twin head and the 

 right trabecula of the left twin head constitute the inner or adjacent elements in question. 



