rOEMATION AND FATE OP THE PRIMITIVE STREAK. 495 



The mere fact that in Rana temporal' ia the distance 

 from the dorsal margin of the blastopore to the point at which 

 the epiblast is separated as a distinct layer remains constant 

 throughout the earlier stages is not a proof that the blasto- 

 poric lips undergo no concrescence from before backwards, but 

 it is a proof that the epiblast after its separation does not again 

 fuse with the subjacent layer. 



The Formation of the Coelom. 



Along the line of attachment of the mesoblastic plates to 

 the hypoblast, at the sides of the chorda, it is possible to find 

 at irregular intervals small evaginations from the archenteron 

 (fig. 6, D), but these small diverticula cannot be traced for any 

 great distance into the mesoblastic plates. 



They remain for a time as blind diverticula, and then they 

 disappear. So far as we have been able to discover they do 

 not communicate with the coelom, and the coelom is not 

 formed by extension of the archenteric diverticula, as in 

 Amphioxus, but by a splitting of the mesoblast which first 

 occurs laterally, and then extends dorsally and ventrally as in 

 the higher Vertebrata. 



Alphabetical List of Literature referred to. 



1. Balfour, F. M. — * Comparative Embryology,' vol. ii, London, 1881. 



2. Balfour, F. M., and Deighton, P. — " A Renewed Study of the Ger- 



minal Layers of the Chick," ' Quart. Journ. Micr. Sci.," N. S., vol. 

 xxii, pp. 176—188. 



3. Van Beneden, E. — " Untersuchungen uber die Blatterbildung den 



Chorda kanal und die Gastrulation bei den Saugetieren, Kaninchen und 

 Vespertilis muriuus betreffend," ' Anatomischer Anzeiger,' Jahr. iii, 

 pp. 709—714. 



4. " Developpement enibryonaire de.s Mammiferes," ' Bulletin de rAcademie 



Belgique,' 1875. 



5. Bonnet, R. — " Beitriige zur Enibryologie der Wiederkiiuer, gewonnen 



am Schafei," 'Archiv fiir Anatomic und Physiologie — Anatomische 

 Abtheilung,' 1884, pp. 170-230, plates ix— xi. 



6. Ibid., 1889, pp. 1—106, plates i— vi. 



