16 THE VOYAGE OF H.M.S. CHALLENGEE. 



mature state than the young of the Turtle. The Australian Fowl ( Talegalla lathmni) 

 is retained in the large egg until it is as ripe as the hatching Turtle. 



On the whole, the growth of the parts is so alike, in both Turtle and Fowl, that 

 to one most familiar with the development of the bird, it requires some reflection for 

 the mind not to become unconscious that the structures seen belong to a creature of 

 another and a lower Class. 



Thoroughly Chelonian in outer form, these embryos are yet overloaded with brain- 

 and sense-capsules. The head is very disproportionate to the body, and the eyeballs 

 out of all proportion to everything else (PI. III. fig. 1). 



The neck is now much shorter relatively than in the last stage, but is very thick ; 

 the carapace now forms a large pent-house to the trunk, and the abdominal walls are 

 much flatter, although still very convex. The umbilicus (ilv.) is now fairly in the middle 

 of the abdomen, and its thick rim is relatively much smaller. 



Both the ribs and spines mark the carapace, and its lower edge all round is a 

 thick fold in which the infero-lateral dermal scales will be formed. Fore and aft, the 

 rim of the carapace has overlapped the limb-girdles, the specialisation which, of all 

 others, is most characteristic of the Chelonia ; the regions and rays of the limbs are 

 evident, although covered with skin and muscles. 



The tail is rather long, and coils forwards under the abdomen, so as to reach 

 nearly to the umbilicus, which has just room for its vessels between the tail and the 

 uj)per lip. 



The folds of the neck indicate the old closed seams or clefts ; the first j^ost-oral is, 

 however, utilised, and although covered with skin, is a conspicuous part because of its 

 internal expansion, and the parts that enclose it and grow into it. 



These parts are the curved and hollow "quadrate" cartilage, the modified skin form- 

 ing the memhrana tympani, and the discoidal symplectic end of the einhyal element, 

 now transformed into the columella auris (PI. III. fig. 1, and PL IV. fig. 7, q.,in.ty.,co.). 

 Over these parts the auditory capsule (au.) is visible as an ovoidal swelling ; some dis- 

 tance further forward the eyeball (e.) occupies a large region in the upper face ; and in 

 front of it the nasal organ (ol.) is seen as a rounded eminence perforated at the top. 



Below and between the nasal openings (e.n.) the rostral prickle or spine (r.sp.) forms 

 the actual end of the head, the opening of the mouth is still a good distance further 

 back, and the mandible {mn.) is relatively small and feeble. 



The folded mid-brain (C 2) still preponderates over the other vesicles, and although 

 much further back than in the last stage, is a very tumid region, yet the same thing is 

 seen in the snake at this stage. (See Snake's Skull, 3d stage, Phil. Trans., 1878, pi. 

 xxviii. figs. 1, 2, C 2.) 



(h.) Upper view of head. — The hemispheres (figs. 1, 2, C la) cover the primary 

 vesicle, yet they make but little projection in the roof of the head. The hind-brain 



