CHIMyEROID FISHES AND THEIR DEVELOPMENT. 



found by sections to represent the spiracle.* Further details of the gill region are 

 given in fig. 49". 



In the various figures given of this embryo we note a number of advancing 

 structures : 



(1) The eyes are now well formed, protrude widely from the head, and are 

 provided with a conspicuous lens. 



(2) The region of the snout shows distinct modeling. Olfactory pits are 

 present and are separate from the rim of the mouth. The snout region, it will be 

 seen by reviewing the preceding figures, notably plate vn, figs. 44-46, does not cor- 

 respond to the greatly dilated eminence which forms the cap-like knob surmounting 

 the head. This appears rather in the region of the forebrain, and the writer does 

 not, therefore, agree in the conclusions of Schauinsland (who, however, it will be 

 borne in mind, examined Callorhynchus, not Chimaera) as to the fate of this singular 

 organ. It has, we suggest, the function of providing for the growth of the contour 

 of the antero-dorsal head regionf rather than for the framework of the snout, as 

 Schauinsland suggests. 



(3) The mouth has broken through, and its margins are thickened. It shows 

 distinct movements, although at irregular intervals, in the living young. Between 

 the rim of the upper jaw and the eye appears the spiracle, and in a remarkably 

 anterior position contrasted with that of an elasmobranch. 



(4) The five gill-arches (plate vm, fig. 49 d ) show well-developed lamellae on 

 their anterior margins, and from these are produced the external gills. The latter 

 extend outward on either side to a distance equal to about the diameter of the head 

 between the eyes. The presence of dilated spaces, blood-filled, in the external gills 

 has already been recorded. It is worthy of note, perhaps, that when the present 

 specimen was preserved masses of yolk (plate vm, fig. 49 b ) were found adhering 

 to the gill-filaments, a fact which may have some significance, since the blood- 

 dilated spaces appeared at points adjacent to the attached yolk masses. In this 

 stage, it may be added, the fifth gill-slit has not as yet broken through. 



(5) The fins are well established. The lobe of the anterior dorsal fin, however, 

 shows as yet no trace of a spine. The paired fins are distinct lateral folds, much 

 as in the young shark; in fact, the pectorals are even precociously large. It may be 

 added that the metameral elements of the fins were conspicuous in the living 

 embryo, since blood-vessels were present and appeared in a series of brilliant spots. 

 The ventral fins are drawn together immediately behind the anus, and no trace 

 appears of a clasping organ or of a third pair of limbs. % The general arrangement 

 of the fins is best seen in plate vm, fig. 49. 



(6) The yolk-sac, in spite of its small size, was perfect. Its structure is 

 delicate, for at first its contour was smooth, but after the embryo had been kept 

 living for several hours in sea-water, it was noticed that the surface of the sac 



This is not in the position in which Solger (Morph. JB., 1876, pp. 219-221) expected it to appear, i. e., behind the 

 articulation of the mandible. 



fPossibly as a larval organ to protect the head when in contact with the wall of the egg-capsule. 



\Cf. T. J. Parker, Nature, vol. xxxix, p. 625. With regard to the non-appearance of mixipterygia, which 

 certainly occur early in Chimseroid ontogeny (cf. infra, plate ix, fig. 50^ also text), it is possible, of course, that the 

 present embryo was a female. 



