No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 383 



A later stage is represented, PI. XX, Fig. 22, in which the 

 anterior vesicle is much reduced in size; a slight constriction 

 marks the boundary between the two portions of the germ- 

 ring. Sections of this stage (PI. XXI, Fig. 14) show the blas- 

 topore {bp) to be also reduced in size. The periblast {per) 

 forming the yolk-plug is thickened; so far as I am able to 

 determine the caudal vesicle {c.v.) does not open to the exterior. 



The section Fig. 1 5 represents a stage when the closure of 

 the blastopore is nearly complete; the continued thickening of 

 the periblast is obvious ; within it are a number of minute cau- 

 dal vesicles {c.v), which may represent the remnant of the 

 anterior cavity {c.v.). The large cavity {c.v!) seems to have 

 undergone no farther change during this interval. Its epithe- 

 lial covering is on all sides continuous with the tail-end of the 

 embryo. The size of Kupffer's vesicle has remained practi- 

 cally unchanged. 



The embryo soon grows away from the constricted portion 

 and the condition represented in PI. XX, Fig. 23, results, when 

 there is no longer communication as sections shown with the 

 body of the embryo. Many variations exist in the distance 

 between the posterior end of the embryo and the extra embry- 

 onic portion of the germ-ring depending upon the relative rate 

 of growth of the embryo. 



In an extremely late stage (Fig. 24), when the embryo has 

 almost reached the larval condition, the ring is still present. 

 Sections at this stage (PI. XXI, Fig. 10) show that the size 

 of the posterior vesicle is somewhat reduced. The periblastic 

 ring, however, remains well defined. The section Fig. 1 1 is a 

 more highly magnified picture of the region marked per. in 

 Fig. 10, showing the aggregation of periblastic nuclei in this 

 locality, also the continuous layer of epiblast {ep). Since these 

 are the latest stages in my possession, the fate of this part of 

 the germ-ring remains undetermined. 



The striking similarity of Kupffer's vesicle to the caudal 

 vesicles c.v. and ^r.'Z/.' and the close connection often observed, 

 amounting in some cases (Fig. 14) to almost a complete fusion, 

 lead one to infer that these vesicles have arisen from an orig- 

 inally single vesicle. 



