DETAILS OF LATER EMBRYO. 



107 



is long, narrow, and relatively of great size, suggesting its origin from an anterior 

 fin spine, and interesting in connection with paleontological data(^'. figs. 132-137). 

 In fig. 50^ an idea is had of the extent of the overgrowth of the opercular fold on 

 the ventral side of the head, and here is shown also that the external gill-filaments 

 arise only from the anterior wall of the gill-slit, and that the external filaments 

 increase in length as they pass toward the middle of each flap. A detail of the 

 ventral fin is shown in fig. 50*^. Here the mixipterygium is but a further differen- 

 tiation of the base of the ventral fin {cf. plate viii, fig. 49''), and the anterior clasping 

 organ («c/) evidently represents the fin's anterior segmental elements (radiaha) 

 {cf. also fig. 112). The mouth region 

 in this stage is noteworthy, since it 

 shows that not only are the anterior 

 and posterior dental plates {adp and 

 pdp) present, but also a series of other 

 eminences which are best interpreted 

 as rudimentary dental plates. Similar 

 structures are now described in detail 

 in the work of Schauinsland on Cal- 

 lorhynchus {v. infra). The present 

 figure also indicates the early stages 

 in the curious lip cartilages of the 

 Chimseroid. They arise at the sides 

 of the mouth and suggest at this 

 stage the corresponding structures in 

 shark. In view of the recent work 

 of Schauinsland and of the younger 

 Furbringer (Morph. JB., 1903, vol. 

 XXXI, pp. 360-445), we recognize with interest the unpaired element at the mandib- 

 ular symphysis which is held to represent the homologue of the basihyal of the 

 hyoid arch. {Cf. fig. 1 1 1.) In commenting further upon this stage we note that in 

 the eye the iris is well established, and that in the umbilical sac the yolk material is 

 arranged in conspicuously concentric lamellae (plate ix, fig. 50''). 



The Skull. — The skull at this stage may be compared instructively with that 

 of a late embryo of Callorhynchus figured by Schauinsland in Taf. xvii, figs. 124, 

 125, 126, op. cit. The present figs. 84 a-d were, like the figures mentioned, prepared 

 from wax-plate models. The embryo referred to by Schauinsland is more advanced 

 than the present one, although the difference in age does not appear to be conspic- 

 uous. On the other hand, the figures of a younger Callorhynchus shown in 

 Schauinsland's Taf. xviii, figs. 130 and 131, can not be compared satisfactorily 

 with the present specimen of Chimaera, for its skull was evidently far less mature, 

 a large part of the model having been based upon outlines of procartilage. A 

 study of the foregoing figures indicates that the skull of Chimsera is, at a corre- 

 sponding growth period, the more highly modified ; the orbits are larger, the snout 



Fig. 82. — Transverse section through the region of the ventral fins of 

 precetling embryo. 

 be. Body cavity ; Crt, caudal artery; ct\ caudal vein; "i^ tubule of meso- 

 nephros ; ^'l\ posterior portion of segmental duel. 



Fig. 83. — Transverse section through the tail region of the preceding 

 embryo. 



