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ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



elusions 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 

 portion 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 (concrescence 

 theory). 



2. By fusion of the lips of the blastopore between the neurenteric 

 canal and anus — Schwarz (47). 



3. By fusion of the epiblast and mesoblast from behind forwards 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 already referred 

 to concerning the formation of the anus in Amphibians. 



1. It is a new formation in the region below the blastopore — 

 Balfour (1), Sidebotham (51). 



2. It is the blastopore — Sedgwick (49), Alice Johnson (27), 

 Spencer (52). 



