No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 387 



Concrescence. — Following the observation of Kowalewsky 

 ('7l), who considered the dorsal groove in the Frog, Amphi- 

 oxus, and Ascidians as representing a continuation of the gastrula 

 invagination, the papers of His ('76) on the Teleost and Rauber 

 on the Chick clearly formulated the Theory of Concrescence, 

 the evidence of this process in Amphibia being based upon the 

 statement of Kowalewsky. 



The presence of this groove has been considered by a num- 

 ber of later writers as evidence of concrescence. 



Since I have already dwelt upon this point (p. 376), I refer 

 the reader to the criticism there offered. 



According to Roux the Amphibian embryo is formed by 

 concrescence in the following manner : " Das Material zur 

 Bildung der Medullarplatte jederseits durch seitliches Herab- 

 wachsen vom Aquatorrande aus auf die Unterseite des Eies 

 geschoben wird, und dass diese von beiden Seiten her entge- 

 genwachsenden Flatten unten in der Medianlinie mit einander 

 verschmelzen. Diese Verschmelzung findet successive und 

 zwar in cephalocaudale Richtung statt." 



This conception is based largely upon the experimental de- 

 termination of the fate of the cells at the dorsal lip of the 

 blastopore, which region, according to Roux, represents the 

 head of the embryo.^ If this one observation becomes an estab- 

 lished fact, it will afford very conclusive evidence of concres- 

 cence. It must at present, however, be considered as a tentative 

 hypothesis, since the observations of Schultze ('88), as well as 

 my own experiments are far from confirmatory. 



From a study of pathological forms Hertwig ('92) likewise 

 holds that the embryo is formed by concrescence. " In der 

 Ruckenlinie erblicke ich die Nahtlinie, in welcher bald nach 

 dem Beginne der Gastrulation die Urmundrander sich in einer 

 von vorn nach hinten langsam fortschreitenden Richtung in 

 der Medianebene zusammengelegt haben und verschmolzen 

 sind." I am unable to discover whether Hertwig maintains 

 two altogether different processes in Triton and the Frog, or 

 whether he retracts the belief which he previously stated ('82), 



1 The late experiments by Morgan {Anat. Anz. 1894, p. 698) confirm the 

 observations of Roux. 



