349 



this mark occupied the position shown in Fig. 27 which indicates, with 

 much certainty, that the material lying on either side of the margin 

 of the blastopore is later carried in toward the median line of the 

 embryo. 



24 



We thus observe that the transverse portion of the neural fold, 

 which indicates the position of the cephalic end of the future embryo, 

 arises in a different region from that of other Amphibia (Rana, Bufo, 

 Acris, Chorophilus). If this be true and the level at which the dorsal 

 lip of the blastopore appears is not farther removed from the upper 

 pole than in the other Amphibia; and it certainly is not, we must 

 then infer that either a greater extent of the embryo is formed through 

 a backward extension of the dorsal lip of the blastopore or that a 

 coalescence (concrescence) of the lateral lips of the blastopore plays 

 a more important role. The experiments show that the backward 

 extension is about the same as in the other Amphibia. Moreover 

 experiments 26 — 27 indicate that one half, or more, of the embryo is 

 to be regarded as having formed through concrescence. 



Having shown that a much greater portion of the embryo of 

 Necturus is formed through concrescence than in the other Amphibia 

 we are naturally led to seek for an explanation in the modified character 

 of the egg. The unpublished observations by Whitman and myself 

 show that the cleavage of the egg is quite unlike that of other Amphibia 

 in that, while holoblastic, it approaches more closely the meroblastic 

 type than is the case in any other of the known Amphibia. 



The statement is almost axiomatic that the increase in quantity 

 of yolk, and its progressive segregation from the germinal material, 

 is evidenced by the increasing meroblastic tendency of cleavage. From 



