650 BASHFORD DEAN, 



Larva of the seventh day (PL 10, Fig. 17; dorsal aspect 

 PI. 10, Fig. 18). 

 The chondro-cranium is uearly completed at this stage, giving 

 the frontal region somewhat its adult appearance. The sucking disc 

 is reduced to a mere tubercle and is no longer functional; maxillae 

 and premaxillae are established; the opercula have increased widely 

 in outline, broadly enclosing the front of the yolk-sac region. The 

 mucous canal tracts of the head are now mapped out most favorably ; 

 they have sunken into the skin as whitish lines and are gradually 

 becoming darkened at various points by the overlapping tissue of the 

 margining ledges. The pectoral fins have greatly increased in size 

 and are in constant motion ; they now suggest the archipterygial stage 

 of the adult Poli/pterus, — especially interesting is this in its ad- 

 verseness to Gegenbaur ^ in bis last paper on the Archipterygial 

 theory of the paired limbs, since it has been seeu to have been 

 preceded by the shark-like fin-form, and this in turn by the ptycho- 

 pterygial type in its early dermal fold. At this stage the nasal open- 

 ing, greatly constricted, is becoming divided into its anterior and 

 posterior divisions by a dermal outgrowth of its anterior margin, an 

 ontogenetic stage suggesting the adult conditions of sharks. The en- 

 largement and strengthening of the muscle plates of the trunk should 

 also be noted. The eyes, it is interesting to remark, are for the 

 first time in constant movement, turning about on all sides in the 

 most marked way, — the development of the eye muscles seeming to 

 be accompanied by the need of their constant exercise, for at no 

 later period are their movements so prominent. 



Larva of the tenth day (PI. 10, Fig. 19; jugular region in 

 PI. 10, Fig. 19 a, sucking disc b). 

 From this stage onward the larva has to acquire but the minor 

 characters of the adult. The general body outline has become esta- 

 blished, the yolk sac largely absorbed, its walls now entirely surrounded 

 by downgrowing muscle plates. The head region is distinctly tliat 

 of the adult, its skeletal and muscular Clements clearly modelled; 

 mucous canal openings have now been formed by the process of en- 

 closing the sensory tracts begun in the preceding stage; the mouth 

 has its mature form and movements ; the nasal openings are established ; 



1) Morph. Jahrb., V. 21, Heft 1, p. 119-160. 



