FOEMATION AND FATE OF THE PEIMITIVE STEEAK. 457 



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 — Side- 

 botham (51) ; 

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

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

 (47), Goette (15) ; 

 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 Rauber (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 ob- 

 servations 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 blastopore 

 are approximated and united, that there should result a mesial 

 axial rod of fused tissue, which cannot in the first instance be 

 resolved into distinct layers. Such a fusion of the blastopore 

 lip was long ago shown to occur in Elasmobranchii by Balfour 

 (1, p. 51), who compared the line of fusion of the lips of the 

 Elasmobranch blastopore to that portion of the blastodermic 

 area of the Avian ovum to which the term primitive streak 



