154 CHIM^EROID FISHES AND THEIR DEVELOPMENT. 



If this general position be granted, we have still to consider the question 

 whether Chimaeroids actually possess any primitive characters. Reviewing the 

 materials at hand I think we may here refer to the following: 



I. Holoblastism. The egg cleaves totally. Of this there can be no doubt, 

 although, as we have seen, this condition is complicated in many ways (pp. 58-63), 

 and its retention is with strong probability due to the highly modified physiological 

 needs which it now subserves. In other words, the holoblastism of Chimaera is 

 less primitive than adaptive, and thus may not represent the ancestral condition in 

 cleavage of such a form as the shark Cestracion (Heterodontus). 



II. Gastrulation. The appearance of the blastopore in front of rather than 

 at the rim of the blastoderm is, I take it, of no little significance as a primitive 

 character. Its retention is probably correlated with the survival of a holoblastic 

 type of cleavage. 



III. Primitive conditions in the mouth region. No one, I assume, will deny 

 that a pharyngobranchial element in the hyoid arch is a primitive feature. And of 

 kindred significance are: The presence (i) of copular segments in the branchial 

 arches, (2) of a mandibular copula, (3) of a pharyngobranchial process in the mouth 

 arch, and (4) of more distinct "preoral arches" than in sharks. On the basis of 

 these characters, then and they are clearly of no little weight may we conclude 

 that Chimaera pictures more accurately than shark the ancestral gnathostome? To 

 this conclusion there are clearly two lines of objections. First, that in many other 

 features Chimaera is singularly modified, and, second, that the mouth region of 

 Chimaeroids is the less easily compared with that of recent sharks on account of the 

 autostylism which has prevailed in the former groups since (at least) Jurassic times. 

 In other words, in view of the first objection, it would be judicious, I conclude, to 

 interpret the foregoing remarkable characters in the mouth parts of recent Chim- 

 aeroids in the following way : That autostylism, although in itself a modified 

 condition, tended less to alter the neighboring branchial structures than did the 

 adaptation of a more flexible support for the jaw-hinge (c. g., as in the modern 

 sharks). And that thus, under the partially conservative influence of autostylism, 

 Chimaeroids, in spite of other structural modifications, have nevertheless retained a 

 few of the characters of primitive sharks. 



The foregoing conditions (I, II, and III) are, as far as I am aware, the most 

 important findings of embryology as to the primitive position of Chimaeroids. Less 

 important in this question are the earlier data of morphology (v. pp. 4-5). Thus: 



IV. Absence of ribs. This character becomes of minor importance, in the 

 light of developmental documents. The early shortening of the visceral cavity 

 would obviously be unfavorable to the development of ribs, even if these elements 

 had been present in the ancestral form. As to the latter condition, it may be 

 mentioned that at the present time there is good reason for the belief that in the 

 earliest sharks (Acanthodians and Cladoselachids) ribs were not present. 



V. Stomach, Kidney, Mazza' s Glands. In these structures also the question 

 of primitive conditions is by no means clear. For the early shortening of the 

 visceral cavity may readily have been accompanied by secondary modifications in 

 the viscera. 



