460 ARTHUR ROBINSON AND RICHARD ASSHETON. 



are inclined to believe that he has mistaken the neural furrow 

 for the primitive streak. But if his conclusions are correct, 

 they support the concrescence theory only in part, in so far as 

 the fusion from before backwards of a portion of the blastopore 

 is concerned ; whilst the fusion of the lip of the ventral por- 

 tion of the blastopore, which he describes as taking place 

 from behind forward, is at variance with the concrescence 

 theory. 



Nevertheless, to the summary which has already been given 

 of the fate of the blastopore in Amphibians (p. 457), we must 

 add the further possibility that, according to the concrescence 

 theory, the posterior part of it only is to be recognised in the 

 anus of Rusconi, the lateral lips of the anterior part having 

 undergone a '^ retrogressive fusion " similar to that described 

 by His in bony fishes (22), and that this fusion is continued 

 until the complete obliteration of the blastopore results. 



From the statements noted in the preceding survey of the 

 literature of the subject we may deduce the following summary 

 of the presence or absence and the mode of formation of the 

 primitive streak in Amphibia : 



1. There is no mention of a true primitive streak — Balfour 



(1). 



2. There is a primitive streak situated in front of the anus 



of Rusconi— Schultze (45), Minot (36), Alice Johnson (27), 

 Erlanger (10a). 



3. The fused lips of the blastopore between the neurenteric 

 canal and the anus represent the primitive streak — Schwarz 



(47). 

 The primitive streak is formed — 



1. By retrogressive fusion of the lips of the blastopore (con- 

 crescence theory). 



2. By fusion of the lips of the blastopore between the neu- 

 renteric canal and anus — Schwarz (47). 



3. By fusion of the epiblast and mesoblast from behind for- 

 wards in front of the anus of Rusconi — Schultze (45). 



Before concluding this short survey of previous records we 

 wish to summarise the statements, some of which have been 



