DETAILS OF LATER EMBRYO. 



103 



the yolk as in the former specimen. A more detailed examination of this embryo 

 shows that two gill-slits have broken through. The mouth, moreover, is more nearly 

 completed, the mandible appearing and the visceral region having a more advanced 

 contour. The pronephros is conspicuous. The pectoral fin is present as a longitud- 

 inal dermal ridge. The tail, judging from its twisted condition, is evidently capable 

 of active movements. This, however, in its detailed structure, as shown in a trans- 

 parent preparation (plate viri, fig. 46^) is still distinctly immature; its tip retains 

 a neurenteric canal {nc), and a postanal gut {pag). In the latter the irregularity 

 at the point x is probably artifact. The present embryo measured about 20 mm. 

 in length. 



Fig. 78. — Transverse section passing through the posterior head region of embryo o( plate Vll, fig. 42. 



At the right the section traverses an auditory vesicle and the hyomandibular evaginalion. The latter fuses with 



the ectoderm, which here invaginates, but no opening has as yet been formed. 



Fig. 79. — ^Transverse section through the middle of the trunk region of the preceding embryo. 

 df. Ectodermal anlage of dorsal fin. P". Pronephros. spc. Body cavily. 



hb. Hyomandibular evagination. «"• Subnolochordal rod. 



A third embryo of this stage is pictured in plate viii, fig. 49'' to ^. It measured 

 about 35 mm. in length, and was observed living. It was this embryo whose 

 capsule was taken accidentally on a trawl line during one of the writer's visits at 

 Pacific Grove. As already noted, it was found developing in a creamy fluid. 

 When placed in a watch-glass, its general position and color were as here repre- 

 sented. It lay for a while on its side, its diminutive yolk-sac extending outward 

 from the body and the delicate tail region showing constant undulatory movements. 

 Most conspicuous were the bright-colored vessels on the yolk-sac, which outlined a 

 vitelline circulation obviously shark-like. The visceral cavity showed red through 

 the deHcate wall, and in the gill region there were prominent bead-like dilatations, 

 brilliant in color. One notes the bright red spot under the eye, which was later 



