DOUBLE MONSTROSITY— STRUCTURE, CLASSES I. AND II. 15 



pared with Figs. 86 and 87 on the same Plate, which show a still greater degree of duplicity in the 

 pronephros. 



Alimentary Canal. There are two mouth-openings, but the pharynx and the rest of the 

 canal are single, the only evidence of duplicity being the presence of two air-bladder diverticula. 



Muscles. Of composite muscles, the most important are contained in the median mass 

 mentioned above as underlying tbc twin notochords, and as being innervated by the small additional 

 motor roots of the composite spinal cord. This muscular mass is segmented serially by septa 

 which correspond exactly with the septa of the outer (normal) lateral muscles. In the head-region 

 some small and intricately arranged muscles are found connected with the cartilages which represent 

 the reduced adjacent Meckelian and palato-quadrate bars. These muscles are obviously rudiments 

 of adjacent mandibular and temporal muscles. It has been noted previously, that there are no 

 external recti muscles in connection with the adjacent eyes, and that the superior obliques are either 

 rudimentary or entirely absent. 



Summary for Class I. Union of simple lateral type (p. 6), the twin brains uniting at the optic 

 lobes ; notochords wider apart in the cervical than in the cranial region, and uniting between 

 the 14th and the 20th segments; body thereafter having a normal bilateral structure; skeletal 

 elements in floor of cranium doubled from pituitary region forwards ; doubling less marked in roof 

 of cranium, and practically absent in visceral arch skeleton ; greater doubling in the floor than in 

 the roof of the brain, and in the first part of the spinal cord than in the medulla ; four separate 

 olfactory pits ; inner eyes more or less united ; inner auditory organs not represented ; heart 

 normal ; vessels slightly modified ; pronephros much modified ; gut single, but with two air-bladder 

 diverticula. The chief uniting and composite structures noted are : the trabecular, parachordal, 

 palato-quadrate, Meckelian, supraorbital, neural, and haemal arch cartilages ; the eyes ; the central 

 nervous system from mid-brain to upper end of spinal cord; the inner muscle plates, and the 

 pronephric glomerulus. 



Secondary Fusion. The part played by primary fusion in the very early growth of double 

 monster fish was previously described (p. 6). It was stated also that secondary fusion had to do 

 with the union of tissues already laid down and the attempted production of a more or less normal 

 bilateral condition out of a double set of structures. In Class I., which has just been described, we 

 may trace the working of secondary fusion in the following, among other, characteristics : (1) the 

 practically normal condition of the visceral arch skeleton ; (2) the fact that the roof parts of 

 the cranial skeleton, optic lobes, medulla, and first portion of spinal cord are less modified than the 

 corresponding floor parts; (3) the fact that the first portion of the spinal cord shows greater 

 duplicity than the medulla; (4) the single condition of the pharynx, oesophagus, and heart, as 

 contrasted with the doubling of the air-ducts ; (5) the delayed union of the dorsal aortae, and 

 (6) the double or composite character of the pronephric glomerulus. 



Influence of Sensory Nerves on Growth. Important evidence on this vexed question is afforded 

 by the monstrosities just described. No trace exists of inner or adjacent 5th and 7th cranial 

 nerves. Yet the inner sides of both heads are perfectly well developed wherever they have sufficient 

 space for growth. It is thus evident that, here, the absence of a sensory nerve-supply has neither 

 hindered nor disturbed the natural course of development. 



CLASS II. 

 Union in Head Region, the Twin Brains uniting at the Medulla Oblongata} 



PL I. fig. 6 (external appearance); Pis. XIII.-XIV. figs. 47-51 (transverse sections); 

 diagrams of skeleton, vessels, etc., in PL XVII. figs. 60, 61, XVIII. figs. 65, 68, 71, XX. figs. 82, 86. 



Cranial Skeleton. The structure of the cranial skeleton in this type agrees generally with that 

 in the class last described, except that the place of union of the skeletal elements is carried further 

 back. This gives room for greater development on the part of the inner or adjacent elements in the 



'See note at foot of p. 11. 



