io6 



CHIIVLEROID FISHES AND THEIR DEVELOPMENT. 



The latest embryo in the writer's material, one of the specimens secured by 

 Professor Wilbur, measured 5 1 mm. in length. Its age was said to be six months. 

 (Plate ix, figs. 50 and 50" to g .) It is decidedly like the adult Chimaera, as can be 

 seen from the figures; it has well-established snout (in which sensory grooves and 



pits appear), paired and 

 unpaired fins, and 

 clasping organs, show- 

 ing that the present 

 specimen was a male. 

 On the other hand, two 

 prominent embryonic 

 characters still appear, 

 viz, the yolk-sac (which 

 in the present specimen 

 is preserved only in 

 part) and the external 

 gills, a tuft of which is 

 seen protruding from 

 below the opercular 

 folds. The external 

 gills are shown in plate 

 ix, fig. 50", c , e , and g . 

 Their degree of differ- 

 entiation is indicated in 

 fig. 5o g , in which we 

 note that in each fila- 

 ment one of the compo- 

 nent vessels is less con- 

 torted than its neigh- 

 bor, the filament thus 

 presenting a crinkly 

 appearance when viewed under a low power. Occasionally a terminal dilatation 

 is seen. It will be noted that some of the filaments attain great length, although 

 in general they are fewer in this than in the earlier stage, a process of reduction 

 having set in at certain points.* In lateral view this embryo shows fragments 

 of yolk attached to its side and to its paired fins, a condition probably artifact, 

 although deserving mention, since in the younger stage yolk masses were 

 observed attached to the gills. Before making the present sketch, a portion of the 

 opercular fold and the neighboring external filaments were removed. The sensory 

 canals are well indicated; that of the lateral line has now passed down the side of 

 the body and has entered the tail region. The mandible is well established. In 

 plate ix, fig. 50, we observe the extent to which the opercular folds overlap 

 the tuft of external filaments; we here observe also that the frontal clasping organ 



*Cy. also Schauinsland (op. n't., Taf. xvi). 



Fig. 81. Transverse section passing through the otic vesicles of preceding embryo. 



sides external gills are shown. 

 **, artery; d, dilated blood knot in external gill; *, spiracle; t't vein. 



At the 



