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



shown emerging, and the tenth is cut through at its ganglion. Here we have a nearly 

 perfect section of the occipital arch, the upper part being a little imperfect, and the noto- 

 chord is seen to be bridged over by cartilage in the occipital condyle {iic.,og.c.).^ 



Sixth Stage. Embryos tivo-thirds ripe : total length, 3 inches ; length of head, 7 lines. 

 — (a.) " Chondroeranium." — The primordial or cartilaginous skull is now quite perfect, 

 and some bony tracts have appeared in it. It is now perfectly Chelonian in every respect, 

 and a certain cartilaginous tract, — viz., the alisphenoidal, — which remains in one form or 

 another in other types, has here been absorbed, and this is correlated with the modifica- 

 tion of certain investing bones and visceral bars, such as is seen in no other type. 



This type is, indeed, very instructive, for whilst showing its Chelonian nature very 

 early in the embryo, it retains a number of characters throughout the whole embryonic 

 period that are very generalised for a re]3tile, being such as we see in their perfection in 

 the anamniotic " Anura." 



Thus, whilst these shielded types are the most curiously modified of all the cold- 

 blooded Sauropsida, they are built up, so to speak, upon the foundation of the underlying 

 low Batrachian forms, the stigma and stamp of which they, in spite of their higher nature, 

 never lose, yet they are all conformed to a pattern, as new as it is perfect. Another thing 

 to be noted is this, namely, that the Batrachian characters are developed late, after the 

 skull has undergone all its principal metamorphoses ; or rather, in metamorphosing, these 

 characters appear, last for a considerable time if they are not permanent, and in some 

 things are life-long. 



In like manner the Batrachians themselves, whose descent has to be counted from two 

 lines, namely, from the Lamprey-tribe and the tribe of the Sharks and Skates (Marsipo- 

 branchii and Elasmobranchii), in these the suctorial fish is pre-potent and dominates the 

 larval stage ; but when the later metamorphosis begins, then the higher Selachian characters 

 appear. These, however, are not retained in their simple uncombined form, nor are they 

 kept to their old functions, but they form unwonted combinations for special purposes in 

 these higher organisms. The " old things " of the Lamprey tribe are partly done away, 

 and partly put to new purposes, in new shapes ; partly absorbed, and partly transformed. 



And so it is in the Chelonia and in all ascending and improving types ; the mate- 

 rials are the same as in low kinds, but " the old order changeth, yielding place to 

 new." Of this we may be certain, that if the structure and development of the extinct 

 types could be known — a thing impossible and only imaginable — every modification of 



' The reader is asked to compare the three sets of sections together, and these also with the solid sections and 

 dissections. Many things stndied by me in the sections were not drawn, and others that are drawn are neither lettered 

 nor described, for I have been careful not to overload the subject ; but the chondroeranium, with the most important 

 parts of the nervous centres and nerves, have been selected for portrayal and description. Several such memoirs as 

 the present wonld be needed for an e.xhaustive account of the development of all the organs, even of the cephalic region 

 only, to say nothing of the rest of the organism. 



