FORMATION AND FATE OF THE PRIMITIVE STREAK. 87 



Erlanger's (10a) observations upon the Anura have led him to the 

 conclusion that the blastopore closes both from before backwards and 

 from behind forwards. The closure from before backwards is associated 

 with the formation of the primitive streak, which lies in front of the 

 neurenteric canal. The closure from behind forwards gives rise, in 

 the first instance, to a fused mass of cells, which rapidly differentiates 

 into layers, after which the anus forms in the situation previously 

 occupied by the posterior portion of the blastopore. Therefore, so 

 far only as the statements of the authors to whom we have referred 

 are concerned, the blastopore of Amphibians may either — 



Become (1) gradually constricted, being transformed into a narrow 



canal, which for a time unites neural and alimentary cavities, 



but eventually disappears — Balfour (1), Schultze (46). Scott 



and Osborn (48) ; 

 Or (2) it is transformed into the anus — Alice Johnson (27), 



Spencer (52) ; 

 Or (3) it is not enclosed in the neural folds, it does not form the 



anus, but gradually disappears — Sidebotham (51) ; 

 Or (4) the anterior part becomes the neurenteric canal and the 



posterior part the anus — Morgan (39), Schwarz (47), 



Goette(15)j 

 Or (5) the anterior portion becomes the primitive streak, the 



middle portion the neurenteric canal, and the posterior part, 



after being closed, is again reopened in a small portion of its 



extent as the anus — Erlanger (10a). 

 But this summary does not include all the fates which are allotted 

 to the blastoporic opening, for its history has been intimately connected 

 with that of the primitive streak and the axial line of the embryo by 

 the researches of His (22 and 23) and Kauber (42), whose observations 

 resulted in the " concrescence theory " of Vertebrate development, 

 which Minot (36) has striven to support in his recent papers. It is 

 necessary to consider this theory, as the results of our observations 

 bear upon it ; but before referring further to it we must draw attention 

 for a moment to the structure of the lips of the blastopore. It is 

 almost universally allowed that in all Vertebrates, with the exception of 

 Amphioxus, there is in this region a fusion of all three layers of the 

 germ, and it follows as a consequence, if the lateral borders of the 



