48 THE VOYAaE OF H.M.S. CHALLENGER. 



Second Stage. 6 lines (^ inch) long. — The number of somatomes has greatly increased ; 

 the three divisions of the heart are fairly formed ; the rudiments of the sense-capsules are 

 more distinct, that of the ear still showing the opening ; the maxillo-palatine fold is seen 

 in the form of a sessile pointed leaf, the base of which is attached to the top of the 

 mandibular fold, the latter being more than twice the size of the former ; the naso-frontal 

 process is not yet formed ; the pituitary involution is beginning (PI. I. fig. 2). 



Third Stage. 6^ lines long. — The number of somatomes has now increased to about 

 fifty-one, there being only about forty-one vertebrse in the adult, which thus aborts 

 seven segments in the cervical region, and three in the caudal, there being the same 

 number in the dorsal, lumbar, and sacral regions, both in these embryos and in the 

 adult. In the latter parts, the somatomes form a sharp inferior edge above the upper 

 margin of the limbs, which is the rudiment of the carapace. The limbs are rounded 

 paddles, attached to a broad base, and the pectoral member is midway between the nose 

 and the tail. The ear oj^ening is covered with skin ; rudiments of the naso-frontal 

 process are seen ; the maxillo-palatine has increased fourfold, and has, like each of the 

 post-oral folds, a distinct opercular fold projecting over the cleft behind. Head cavities 

 can be seen in the visceral folds ; the pituitary involution is more distinct ; the notochord 

 reaches nearly to the top of the fold of the mid-brain (middle trabecula), and is curved 

 over and enlarged at the end (PI. I. figs. 3-6). 



Fourth Stage. 9 lines long. — The number of somatomes is the same as in the last 

 stage ; the rudiment of the carapace is very distinct ; the head is larger than the whole 

 thoracic region of the body, and rudiments of the hemispheres are apparent in front of 

 the pineal elevation ; the mid-brain is very outstanding, and the hind-brain is much 

 hidden by lateral growths ; the maxillo-palatine, is twice as large as the post-oral, folds, 

 which are now contracting upon all the .clefts ; the Eustachian openings are wide apart. 

 The quadrate is already sickle-shaped, enclosing the bulbous distal end of the columella in 

 the rudimentary membrana tympani, which closes up the upper part of the cleft. There 

 are distinct lachrymal and nasal clefts, and between the latter a dUated, rounded naso- 

 frontal process is seen, on the centre of which is the rudiment of the rostrum for breaking 

 the shell. The racemose pituitary body has not yet united with the infundibulum ; the 

 olfactory lobes are quite distinct from the solid olfactory nerves, and the optic nerves 

 are hollow. A chondrocranium is already formed, and the basis cranii runs high into 

 the fold of the mid-brain, forming the post-clinoid wall, the notochord runs nearly as 

 high as this latter, and is clubbed, and turned downwards. From the pituitary region are 

 seen the broad trabeculse segmented from the investing mass, and a long intertrabecular 

 bar ending in the prenasal rostrum ; from the trabeculce grow the large orbito-sphenoids, 

 which lie low down ; the orbito-nasal septum is not developed (Pis. I. and II.). 



