624 J. PLAYFAIR MOMUREICH. 



A. Length 3 — 4 mm. : cartilages not quite fully formed. 



B. Length 6 — 7 mm.; cartilages fully formed; considerable 

 amount of yolk still present. 



c. Length 8 — 9 mm. ; yolk-sac very much diminished in 

 size. 



D, Length 10 — 11 mm.; yolk completely absorbed; ready to 

 leave the brood-pouch. 



E. Adult. 



I. Development and Structure of the Cranium. 



In Stage a (PI. XLII, fig. 1) the head is round and small, 

 with the lower jaw bent up, and closely applied to the under 

 and anterior surface of the cranium, there being as yet no dif- 

 ferentiation of the upper jaw and snout. Below the medulla 

 oblongata (M. O.) the notochord (N.C.) bends abruptly 

 downwards, and becomes ensheathed on either side by the 

 parachordal cartilages, which are continued forward to unite 

 with the trabeculae cranii (Tr. Cr.), the extremity of which 

 is somewhat bent upwards, and as yet, as it were, within the 

 cranium, reaching no farther forwards than a point between 

 the eyes. The auditory capsules (Au.) are formed in carti- 

 lage, and are apparently united to the parachordals, but possess 

 no rudiment of the semicircular canals, though two otoliths are 

 plainly visible. 



In Stage b (fig. 2) the snout makes its appearance, but it is 

 as yet bent upwards, and lies closely applied to the front of the 

 cranium, owing to the upward flexion of the trabeculae men- 

 tioned above, which has increased considerably. At their co- 

 alesced extremities, and articulating with their sides, is on 

 either side a cartilage (E. Pa.), whose signification will be 

 discussed when treating of the visceral arches. When the 

 skull in this stage is viewed from above (fig. 4), the nares (01.) 

 may be seen lying on the upper surface of the snout, slightly 

 anterior to the eyes. They have a thickened margin, and be- 

 hind them, extending down from the centre towards the sides 

 of the snout or rostrum, are two cartilaginous bars (Na. C), 



