DOUBLE MONSTROSITY— STRUCTURE OF NORMAL EMBRYOS 11 



another in front. Two heads were thus formed, but in later development one of them remained 

 as a rudiment, forming a knob attached to the other and containing traces of an eye. 



Nothing is more striking in these and other observations by Lereboullet, than the amount of 

 what may be called secondary fusion which can take place between closely approximated twin sets 

 of structures, provided that they come together at a sufficiently early stage. For example, two head 

 rudiments may, in the end, be converted into a single apparently normal head devoid of inner eyes 

 or olfactory pits. Even two inner series of composite niesoblastic somites may unite, become 

 absorbed, and disappear. Doubling of the notochord, however, seems never to be recalled, though 

 the twin notochords may come to lie close against one another through absorption of the intervening 

 tissue. The period when the power of fusion becomes lost in the case of mesoblastic somites is near 

 the time of appearance of the heart. 



ANATOMY OF NORMAL TROUT EMBRYOS. 1 



PI. I. fig. 1 (external appearance); III. figs. 13-1 G, IV. figs. 17-20, VI. figs. 24-26 (transverse 

 sections); V. figs. 21-23 (horizontal sections); diagrams of skeleton, heart, etc., in PI. XVII. figs. 

 5G, 57, G2, XVIII. fig. 69, XIX. figs. 73, 76, XX. fig. 83. 



In normal trout embryos of the same age as the monstrosities, the cartilaginous skeleton has 

 long been laid down and the process of ossification is about to commence. 



Cranial Skeleton. The parachordal cartilages are uniting round the anterior part of the 

 notochord and have already joined with the trabeculae cranii, which, coalescing in front of the 

 pituitary space, run forwards as a median flattened bar to meet the nasal cartilages. The 

 pituitary space gives passage to the choroidal and internal carotid arteries and to the back part 

 of each rectus oculi externus. On either side, the parachordals have grown upwards in the form 

 of laminar plates, which meet in the mid-dorsal line over the upper part of the medulla, but leave a 

 narrow V-shaped fontanelle over the lower part. The auditory capsules are firm bosses of cartilage, 

 moulded on the labyrinth, closed externally, but widely open towards the brain. Dorsally, they are 

 connected with each other by a thin vault of cartilage roofing the cerebellum ; anteriorly, they are 

 continuous with the supraorbital bars to be afterwards described ; ventrally, they join the trabecular 

 and parachordal cartilages; and externally they articulate with the hyomandibulars. The 5th 

 nerves emerge through deep grooves between the trabeculae and the auditory capsules, while the 

 vagus and glosso-pharyngeal nerves pass out together through a foramen in the cartilage connecting 

 the auditory capsules with the parachordals. A single opening in the floor of the capsules on either 

 side gives passage to the internal jugular vein and the facial nerve. The nasal cartilage is 

 connected with three pahs of bars: (1) the trabeculae cranii, (2) the palato-quadrates, and (3) the 

 supraorbitals. These last pass backwards along the dorso-lateral angles of the brain to join the 

 anterior part of the auditory capsules on either side. Over the pineal body and the third ventricle 

 the supraorbitals are connected together by a bridge of cartilage, but no such tegmen exists over the 

 cerebral and the optic lobes, the spaces left uncovered being the anterior and the middle fontanelles. 



Visceral Arch Skeleton. The hyomandibulars are connected with (1) the outer aspect of the 

 auditory capsules, (2) the posterior ends of the palato-quadrates, and (3) the interhyals. The 

 Meckelian bars are slender, and meet below the mouth in a symphysis. Posteriorly they articulate 

 with the palato-quadrates, but not with the hyomandibular or interhyal cartilages. In the hyoid 

 arches, glossohyals, hypohyals, ceratohyals, and interhyals can be distinguished. The branchial 

 cartilages are five in number, and have the usual forms and relations. 



Pectoral Girdle, Notochord, etc. The pectoral girdle is represented by a comparatively short bar 

 of cartilage on either side — the coraco-scapular — and is far from being a complete arch ventrally. 

 The limb-cartilage is an unsegmented plate continuous with the coraco-scapular bar. The notochord 

 consists of pith-like tissue surrounded by a very firm capsule, and its anterior end is embedded in 



'Full references are given in the Index of St>-uctures, etc. (p. 63) to the various Plates and figures in which iudividua 

 structures receive illustration. 



