The Embryology of Heterodontus japonicus 



735 



In Figure 16, plate I, there are some incomplete intersegmental grooves marking off 

 three or four (probably four) pairs of incomplete somites in front of those that are com' 

 plete. The anterior portion of the notochord is slightly obscured by the inner limb of the 

 left neural fold; but it appears to extend forward farther than the first intersegmental 

 groove, which is quite distinct. It is difficult to identify accurately all the limits of the 

 primary brain vesicles in this drawing; but it seems fairly obvious that the first inter- 

 segmental groove marks the posterior end of the midbrain. In none of the following 

 drawings do we find the somites extending so far forward, though the series often ends 

 anteriorly with one or two incomplete somites. The region of incomplete somites in 



Text-figure. 52. 



Diagram showing the relation between head somites and body somites in a larval Squalus 



acanthias. The somites that degenerate in ontogeny are indicated by broken lines. 



Id, dorsal moiety of the first myotome; Iti.ventral moiety of the first myotome; 2d, 2v, dorsal and ventral moieties 



of the second myotome; 3v, ventral moiety of the third myotome; 7, seventh myotome; a., anterior cavities; 



hyp.m., hypoglossal musculature; M., mouth; ot., otic capsule; sp., spiracle; thr., thyroid. 



After Neal, 1918, Fig. 10. 



Figure 16, plate 1, probably coincides with the four somites that, in Squalus, degenerate 

 during ontogeny. These have been figured by Neal (1918) in a diagram reproduced herein 

 as Text'figure 52. The occurrence, in sharks, of four anterior somites that subsequently 

 disappear is doubtless of evolutionary significance, indicating a metameric origin of the 

 posterior part of the cranium. These somites serve also to connect the anterior head 

 somites with the body somites and thereby establish their serial relationship. 



Before proceeding further with the account of the external development of the 

 Japanese Bullhead Shark, it seems advisable to consider what is known concerning 

 the internal changes during gastrulation in Heterodontus. 



The internal structure of some embryos of H. phillipi in early gastrula stages has been 

 studied in serial sections by Has well (1898). Three of his figures are reproduced as my 

 Text-figures 53 to 55. Before considering these stages directly, it is necessary to describe 

 some preparations for gastrulation made by the advanced blastula. 



