The Embryology of Heterodontus japonicus 



743 



of the trunk (Text'figure 52, page 735). In Elasmobranchs the head somites, like the 

 trunk somites of vertebrates generally, are at first hollow and their cavities communicate 

 with the primitive coelomic cavity. In the head, this communication is by way of the 

 pharyngeal or visceral arches (mandibular, hyoid and branchial arches), as shown for 

 Torpedo in Text-figure 57a. These channels quickly close (Text-figure 57b) and the 

 somites become solid structures. 



Verbindung des 



Kiemenbogen- 



coclonib mit dem 



Cavum pericardii 



Text 'figure 57- 



Diagrams showing the origin of eye muscles, and the extensions of the primitive coelomic 



cavity into the gill-arches, in selachian embryos. In A, the cavities of the pharyngeal 



arches are shown communicating with the pericardial portion of the coelomic cavity; in B, 



which is a later stage, the connections of these cavities have been lost. 



1, 2, 3, 4, gill-clefts; S.B.C.l — 5, pharyngeal arch extensions of the coelomic cavity; ch.dors., chorda dorsalis; 



oc.tn., anlagen of the oculomotor muscles; M. ohliq. suf>. and M. rect.lat., anlagen of the superior oblique and lateral 



rectus muscles respectively; lies. audit., otic vesicle. 



After Corning, 1925, Figs. 222 and 22J; based on Froriep's (1902) Figs. 4 and 5 {Torpedo ocellatus). 



Figure 27, plate II, portrays a cleared embryo of about 41 complete somites. Like 

 the preceding figure, it appears to have been drawn under unusually high magnification. 

 This embryo exhibits a moderate caudal and a pronounced cervical flexure — the latter 

 presumably unusual for this stage since it does not appear in the stages immediately 

 following. The optic cup shows, more distinctly than heretofore, the outer as well 

 as the inner layer. The otic vesicle is larger, and nearer the branchial region; it is dorsal to 

 the first gill-cleft. The bulbus cordis and the ventral aorta are distinctly outlined and the 

 latter has given rise to the first three aortic arches. The hind-gut is outlined faintly along 

 its ventral border and at both anterior and posterior ends of its dorsal border. The 

 neurenteric canal is well shown. 



