SPECIALIZED CHARACTERS. 153 



small blastomeres, nierocytes and undivided yolk — and in the periphery of the 

 blastoderm. We find further that amitosis occurs plentifully within the blastoderm. 



VI. The fragmentation of the egg, which begins at gastrulation, doubtless 

 arose as a primitive character, /. c, holoblastism. Its function, however, in the 

 modern Chim?eroid has become a distinctly complicated one. By this process a 

 large part of the yoW is diverted from its primitive use and is appropriated by the 

 embryo secondarily, via gills and gut. The yolk-sac, accordingly, is reduced to 

 miniature size. 



VII. The embryo develops precociously. While still minute in size, i. e., 

 in terms of the blastoderm, it presents complicated structures; when 2.5 mm. in 

 length it has already 25 somites, and suggests the adult. Compared to the young 

 shark it is also more specialized in its relation to the germinal yolk and in the 

 development of the vascular system. In this connection note also the differen- 

 tiation of many types of merocytes, and the evidence that megaspheres are not 

 primitive ova. 



VIII. The head region of the embryo indicates precocious specialization. 

 We thus note the earlj' appearance and great size of the eyes, the appearance of 

 the cephalic "hood," the greatly shifted position and the reduced size of the 

 spiracle, the condition of the head mesoblast, the fewer and larger gill lamellae, the 

 moniliform character of the external gills, due to the presence of special blood- 

 producing organs, the reduction of the fifth gill, and the early differentiation of the 

 branchiostegal flap. 



IX. The trunk region bears similar testimony in the matter of precocious 

 specialization. We thus observe the early period at which the greatly elongated 

 tail is produced, the anterior position of the anal region even in early embrj'os, the 

 speedy obliteration of the lumen communicating between myo- and splanchnocoele, 

 the early appearance of the dorsal fin-spine and of mixipter3'gia, the last a feature 

 worthy of especial comment, since it indicates the appearance of secondary sexual 

 characters in even small embrj-os. Also to be noted is the great size early assumed 

 by the paired fins. 



X. Larval characters are also developed prominently. To be mentioned in 

 this regard are: Larval coloration; larval proportions of head, trunk, and fins; 

 appearance of greatly enlarged dorsal scales; larval dentition, in which the outer 

 rims of the dental plates become specially developed. 



The foregoing are but the most conspicuous characters to be selected from the 

 present embryological materials. Nevertheless there can be, I believe, no valid 

 question as to their significance; for in no essential regard can they be interpreted 

 as representing conditions so unmodified as to have given rise to the present condi- 

 tions in the development of sharks. * 



*One might, it is true, regard the modern sharks as arrested " larvae " of Chimaeroids, and thus maintain that shark 

 embryos exhibit less complicated conditions than their ancestral forms. But if the diversity of specialized characters, 

 as shown in the foregoing summary, is duly considered, this extreme view, it seems to me, can only fall of its own 

 weight. For in view of the many lines of specialization of Chimaeroids, it seems about as improbable that these 

 forms could have represented the ancestral sharks as that a bird could have represented the ancestral reptile, or that a 

 recent horse could have been the progenitor of Protohippus. 



