4 THE VOYAGE OF H.M.S. CHALLENGER. 



The attenuated tail is coiled and serpentiform ; the swollen and huge head is so bent 

 that the mid-brain lies in front, and the fore-brain below ; the hind-brain is a long, sub- 

 arcuate, compound region, going back to the first evident somatome or muscle-plate. 



In this small embryo — not much larger than the larva of the cheese-fly — the body- 

 segments are already divided into three well-defined regions, namely, cervical, dorsal, 

 and caudal. 



Of the fifty-two recognisable muscle-plates, the first dozen are closely packed, and the 

 next are twice as wide. These latter overlie the rudiments of the axial part of the great 

 trunk ; and the clear selvedge seen below this second region is the line above which the 

 infero-lateral bones of the carapace will be developed, these lying below the ribs and their 

 dermal outgrowths. 



The space from the front of the huge heart (h.) to the interspace between the umbilical 

 vessels (ilv.) and the tail, will be enclosed by the nine bones of the plastron. These are 

 mere parostoses, for the Chelonian, like the fish and serpent, has neither sternal ribs nor 

 sternum ; it has only four of the pairs of bones and one of the unpaired bones of the 

 abdominal region of the " Plesiosaur." 



Half of the ventral region is now taken up by the heart and its pericardium {h.,pcd,), 

 and the hind part of the space is open below, showing the Wolfiian bodies ; behind, the 

 umbilical vessels (ilv.) are seen emerging. 



The somatomes have their lower margins all in a row from the space between the 

 ear-sac and visceral clefts {au.,cl.) to the end of the tail, the coiled part of which occupies 

 the space between the right and left jjelvic limbs. 



The region of the trunk is indicated by the presence of the limb-buds {pc.m.,pv.m.) which 

 grow out from a continuous ridge directly below the segmental muscular masses, and 

 which is separated from that ridge by a clear line of depression. 



This line, and the buds that sj)ring from it in two places on each side, are due to a 

 thickening of the mesoblast at the upper part of the somatopleure. 



The very sharply-defined groove below the broad segments of the rudimentary trunk 

 marks the lower outline of the carapace (see next stage, fig. 7), which will in time so 

 develop as to form a pent-house to the limbs, whilst the dermal plates in its lower 

 edge will articulate, by sutures, with the symmetrical bones of the lower or abdominal 

 plane. 



At present, the only clear Chelonian stigma is this beginning of the lower edge of 

 the carapace ; the limbs are on the outside, as in non-shielded types. 



Now, we can clearly see that the girdle is only the upper dorsal end of the limb, this 

 free part is a roundish thick Jin, separated from the body by the axillary or the inguinal 

 space, as the case may be. 



The development of the intercalary vertebral rings in the massive cervical region, 

 with its crowded muscle-plates, will place the heart at an increasing distance fi"om the 



