52 DEAN. [Vol. XL 



merocyte zone has occurred in the middle of the floor of the 

 segmentation cavity. 



(3) Comparison of the gastrulas {cf. p. 39) ; the rim of the 

 blastopore becomes the germ ring ; the "ventral mesoderm" 

 of H. V. Wilson, the primitive hypoblast of the blastopore's 

 ventral lip : of both lips the marginal fusion with periblast- 

 yolk is in Teleost obliterated, — probably on account of the 

 enlarged size of the yolk, and the more perfect relations of 

 periblast to embryo. The interpretation of the Teleost gastrula 

 of Ziegler becomes accordingly slightly modified : coelenteron 

 extends under the rim of the blastopore from the free end of 

 the "ventral mesoderm" to that of the "primitive hypoblast" 

 of H. V. Wilson. 



(4) The presence of Kupffer's vesicle in Acipenser, v. p. 42. 

 But it is especially significant that the nearness of Ganoids 



to Elasmobranchs in their adult conditions is also to be em- 

 phasized in their early development. In Lepidosteus the 

 shark-like characters in partial segmentation, the formation of 

 merocytes, the mode of origin of the germ layers, have already 

 been noted. Of Acipenser the conditions, not v^^idely different, 

 have retained in addition a neurenteric canal. In this com- 

 parison it is now evident that difference in size of egg or in the 

 character of its membranes cannot be adduced as an insur- 

 mountable objection. Laemargus has shown that an egg cap- 

 sule is not infallibly an elasmobranchian feature ; and by an 

 analogy the difficulty in accounting for the increase or decrease 

 in the number of eggs (as urged for example by Beard ^) seems 

 practically solved in the case of Bdellostoma and Petromyzon, 

 forms universally regarded as of close genetic kinship. In 

 Bdellostoma ^ the few and large encapsuled eggs, moreover, are 

 probably extremely meroblastic. 



The nearing of the phyla of Ganoids and Elasmobranchs on 

 the evidence of early developmental characters seems worthy 

 of especial consideration. Morphology would long since have 

 established the stem of the sharks as most primitive and ances- 

 tral of existing gnathostomcs, had not marked differences been 



1 Anat. Anz., 1890, pp. 146-159 and 179-188. 



2 Ayers, 1894. Lectures of Woods IIoll Marine Biological Laboratory. 



