The Embryology of Heterodontus japonicus 723 



of certain characters which are not known to have any vital significance." As an example, 

 he cites the ''orange spot" which forms such a striking feature of the egg of an elasmo' 

 branch in its early stages. This, in HaswelPs opinion, has been handed down with little 

 change from Paleozoic times. The evidence for this view is not given, but it probably rests 

 on the fact that the "orange spot" appears in the eggs of genera that have been segre' 

 gated in different families from a very early period. 



It is not from surface features alone, nor from early stages alone, that one should 

 look for developmental characters linking Heterodontus to the most primitive elasmo- 

 branchs. In studying the phylogenetic relationships of the various groups of vertebrates, 

 the later stages of embryonic development often yield evidence more satisfactory than 

 anything the early stages afford. In Heterodontus, the field of organogeny is largely 

 unexplored. There are, to be sure, a few contributions that deal wholly or incidentally 

 with the development of organ systems in Heterodontus: such as those of Osburn (1907) 

 on the origin of paired limbs; Luther (1909) on the musculature innervated by the trigem' 

 inal nerve; and de Beer (1924.1, .2 and. 3) on the development of the head. Since it does 

 not lie within the province of the present article to review the literature on organogeny, 

 no attempt has been made to make this list complete. 



THE EGG AND ITS MEMBRANES 



The orientation of the early blastoderm of Heterodontus phillipi within the egg 

 capsule has been described by Haswell (1898). He states that the blastoderm, which 

 appears as a circular reddish-orange spot about 2 mm. in diameter, occupies a constant 

 position in the egg: it is always situated much nearer the broader end of the egg shell. 

 The extremity of the blastoderm destined to become posterior is always directed away from 

 the broader end of the egg shell. This indicates that the egg is anchored by the albumen 

 in such fashion that it is free to rotate only about an axis that corresponds to the long 

 axis of the capsule. In the egg of Pristiurus, the germinal disc is always situated at the 

 pole of the egg which is near the rounded end of the egg capsule (Leydig, 1852). 



In Dean's article entitled ''Reminiscence of Holoblastic Cleavage in Heterodontus 

 {Cestracion) japonicus'\ published in 1901, there is some ambiguity in his use of the term 

 "animal pole". This difficulty arises in part from our preconceptions, for certain features 

 of the egg during early development are apparently unique. In his later manuscript 

 Dean seldom uses the term "animal pole", thereby attaining greater clarity in his descrip' 

 tion of the egg, which follows : 



The egg [of Heterodontus japonicus] measures from 40 to 50 mm. in diameter. It is pale 

 greenish-yellow in color, but bright red in the germinal spot [Figure 79, plate VII]. It is of 

 semifluid consistency, as in sharks generally, and can be removed unbroken from the capsule 

 only with the greatest difficulty. It is enclosed in a glistening, somewhat firm vitelline 

 membrane, and supported by viscid albumen, which in turn is attached to the stout capsule. 

 The orientation of the egg is conditioned by gravity, the germinal area [equivalent to the 

 "orange-yellow spot" of other Elasmobranchs] remaining near the upper pole. It probably 

 does not take its position [precisely] at the upper pole although this was not decided, since as 



