No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 38 1 



much later than those in which the origin of the mesoderm has 

 been described, and when there exists in the axial region the 

 two sheets of so-called "gastral mesoderm." 



7. Gastrulation and Fate of the Germ-Ring in Amiurus and Lophius. 



If the two eggs be compared just before gastrulation, we 

 observe a decided difference in the relative amounts of blasto- 

 dermic and yolk material. In Amiurus the mass of the blasto- 

 disc is to the mass of yolk as i : 8. In Lophius the relative 

 proportions would be as i : 20. As the blastoderm extends 

 there is a thinning of its central portion and a thickening and 

 infolding of its margin. In a certain region the infolding is most 

 pronounced, giving rise to a projecting tongue (Fig. 17) which 

 first defines the long axis of the embryo. A point directly 

 behind, at the edge of the thickening, will be henceforth referred 

 to as the provisional hind-end of the embryo. We may also 

 speak of the thickened blastodermic rim as the germ-ring. 



Passing to later stages of Lophius (Fig. 18), we observe the 

 first outlines of the embryo, terminating at its provisional hind- 

 end in a caudal knob {c.k). A dorsal groove, wanting in many 

 Teleosts, is here present, extending from the anterior end of 

 the embryo to a point just in front of the knob. 



In Amiurus, Fig. 1 2, the head-end of the embryo is outlined ; 

 posteriorly, however, it is not yet defined. The dorsal groove is 

 well marked and communicates with the blastopore. In this 

 form the caudal mass is absent ; there are, however, slight prom- 

 inences on either side of the dorsal groove (Fig. 13) which may 

 be its representatives. The discovery of this bifid condition of 

 the posterior end of the embryo of Amiurus is due to Miss 

 O' Grady, of Vassar College. It is of importance, as it is addi- 

 tional evidence of concrescence. 



Later stages of Lophius (Figs. 19, 20) show the gradual 

 extension of the germ-ring over the yolk and a corresponding 

 differentiation and extension of the embryo; the dorsal groove 

 has disappeared and the optic vesicles are outlined. 



In Amiurus, Fig. 14, at a stage when relatively the same 

 amount of yolk remains uncovered, we find the embryo less dif- 



