152 CHIM.ItROID FISHES AND THKIR DEVELOPMENT. 



(5) Shark-like morpIioIoi::ical characters of early Chiiiurroids : 



The earliest definitely known Chimseroids were clearly shark-like. i\\ this 

 regard attention need only be called to the facts: (i) That they had shark-like 

 dermal denticles scattered over the body; (2) a male clasping organ in the form 

 of a selachian fin-spine; (3) rudiments of vertebral centra in the postoccipital 

 region; and (4), in one form at least, tritors in the anterior dental plates which 

 in arrangement resemble strikingly the teeth of a Cestraciont shark. 



Furthermore, the earliest Chimseroids present no characters which can be 

 fairly interpreted as more primitive than those of sharks. They were, on the con- 

 trary, more modified. Thus in their males they had already evolved the three sets 

 of clasping organs. 



EMBRYOLOGICAL EVIDENCE IN FAVOR OF THE VIEW THAT CHIM/EROIDS ARE DERIVED 



FROM SELACHIAN ANCESTORS. 



The riddle of Chimseroid development can, I am convinced, be read in only 

 one way ; for the evidence yielded by the various phases of embryology points to 

 the modified nature of Chimseroid descent: That is, if we grant the value of tran- 

 sitional stages in demonstrating the descent of the more complicated from the less 

 complicated type, we may in the present case obtain a mass of evidence which 

 must, it seems to me, be regarded as conclusive. The scope of this evidence is 

 seen in the following summary: 



I. Chimasroids are more complicated than sharks in sexual characters. 

 Males differ from females to a greater degree in point of size and proportions, and 

 in the development of clasping organs. Of the latter, sharks have only mixip- 

 terygia, while Chimsroids add to these the antero-ventral claspers (which are 

 modified anterior radials of the ventral fin) and the frontal organ (which is inter- 

 preted as a transposed fin-spine). 



II. The egg-capsule of the Chimseroid is the more complicated. It is larger 

 in proportion to the size of the fish, and is adapted more especially to the needs of 

 the young fish. In this regard we recall its remarkable regional differentiation 

 {i. e., for head, trunk, and tail of the young fish), breathing pores, opercular valve, 

 and organ of attachment— characters more complicated than in the egg-capsules of 



sharks. 



III. The early egg membranes are more complex than in sharks. Here we 

 refer to the changes in the tunic and the behavior of its nuclei. 



IV. The phenomena of fertilization. As one instance of complexity in 

 Chima;ra we recall that following polyspermy, the sperm merocytes divide at once 

 amitotically; while in shark amitosis is attained only after a decadent series of 

 mitotic divisions. Witness also, in the Chima^roid, the peculiar features of the sperm 

 track and the character of the asters. 



V. Early cleavage lines, as in the case of the (highly modified) rays, are 

 suppressed, and the synchrony of segmentation is soon lost. Further complication 

 in Chimsera appears in the germinal wall— in which are confused yolk-masses, 



