RARE FORM OF THE BLASTODERM OF THE CHICK. 387 



this is only a special closure which follows the main or somatic 

 closure. 



Balfour's first ohjection is open to criticism from still another 

 point, since it is based on a feature which he himself has else- 

 where declared to be exceptional. In his ' Monograph on the 

 Development of the Elasmobranch Fishes/ p. 84, he says : — 

 " The only feature in any respect peculiar to these fishes is the 

 closing of the medullary canal, first commencing behind, and 

 then at a second point in the cervical region. In those verte- 

 brates in which the medullary folds do not unite at approxi- 

 mately the same time throughout their length, they appear 

 usually to do so first in the region of the neck." 



The introductory phases of concrescence in the Elasmo- 

 branch are well shoAvn in figs. 1, 2, and 3, copied after His ; 

 and they appear to me to meet the first objection with refer- 

 ence to the angle of coalescence. 



The second objection claims that concrescence subsequent to 

 the establishment of the neurenteric canal would be a simple 

 impossibility. At first sight this appears to be a fatal argu- 

 ment against the concrescence theory. Since the general im- 

 portance of this canal was first insisted on by Kowalevsky, a 

 similar structure has been discovered in the Birds by Gasser, 

 and in the Reptiles by Kupffer and Benecke ; and indistinct 

 traces of it in Mammalia have been reported by Lieberkiihn. 

 There is still much difference of opinion, not only in regard to 

 the meaning of this structure and its relation to the allantois 

 and archenteron, but also in respect to its position. Balfour 

 places it just in front of the primitive streak; Kupffer and 

 Benecke contend that it is situated at the posterior end of the 

 streak; Strahl says it arises near the middle; and finally, 

 Braun finds several independent canals that appear one after 

 the other. Strahl's recent paper (No. 47) is the only one that 

 offers any assistance in meeting Balfour's argument. In 

 Lacerta agilis Strahl finds a small inconspicuous primitive 

 streak, at the centre of which the neurenteric canal first makes 

 its appearance. But the point of chief interest here is that 

 this canal, according to Strahl, travels slowly back- 



