FORMATION AND FATE OF THE PEIMITIVE STEEAK. 499 



52. Spencee, W. B. — "Some Notes on the Early Development of Rana 



temporaria," ' Quart. Journ. Micr. Sci.,' N. S., Supplement, 1885, 

 pp. 123—137, plate x. 



53. Strahl, H. — " Beitrage zur Entwickelung der Reptilien," ' Archiv 



fiir Anatomie und PLysiologie — Anatomische Abtheilung,' 1883, pp. 

 1—43, plate i. 



54. "Beitrage zur Entwickelung von Lacerta agilis," 'Archiv fiir Anatomie 



und Physiologie — Anatomische Abtheilung,' 1882, pp. 242 — 278, 

 plates xiv and xv. 



55. " Zur Bildung der Cloake des Kaninchen-erabryo," * Archiv fiir Anatomie 



und Physiologie— Anat. Abtheil.,' 1886, pp. 156—168, plate iv. 



56. Weldok, R. — "Notes on the Early Development of Lacerta muralis," 



'Quart. Journ. Micr. Sci.,' N. S., xxiii, 1883, pp. 134—150, plates 

 iv — vi. 



57. Wenckebach, K. F. — "Der Gastrulationsprocess bei Lacerta 



agilis," ' Anatomischer Anzeiger,' 1891, pp. 57—66 and 72—77. 



EXPLAN/VTION OF PLATES XXXIV & XXXV, 



Illustrating Dr. Arthur Robinson's and Mr. Richard Assheton's 

 paper on "The Formation and Fate of the Primitive Streak, 

 with Observations on the Archenteron and Germinal 

 Layers of Rana temporaria.^' 



Alphabetical List oj Reference Letters for all the Figures. 



A. Anus. a. Anterior extremity of archenteron. A P. Post-anal gut. 

 AR. Archenteron. AR'. That portion of the archenteron the posterior wall of 

 which is formed by the closure of the blastopore. B, B'. Rim of blastopore lip. 

 b. Dorsal lip of blastopore. B C. Blastoporic canal. BL. Blastopore. BL'. 

 Portion of original blastopore not yet closed. BR. Original position of fused 

 layers, that is to say, lips of blastopore at the time of the first formation of 

 the circular blastopore. BR'. Position of fused layers or lips of blastopore 

 after concrescence to a greater or less extent of the lateral lips of the original 

 blastopore. BR". The dorsal portion of the lateral lips of the blastopore folded 

 over with the neural plate, so that that which was their ventral or inner surface 



