38 ALEXANDER MEEK 



demonstrate that the frog and its allies come into line with 

 the meroblastic Amphibia and with the rest of the terrestrial 

 Vertebrata. It is evident that the segmentation cavity of 

 Petromyzon is not, for the reasons given above, a true segmenta- 

 tion cavity, and evidence is required as to its fate; and this 

 raises the question of the interpretation of the segmentation 

 cavity in other aquatic vertebrates. Even amongst the 

 Urochordates there are examples of the formation of the 

 enteron from an excavation of endoderm. 



Summary. 



The examination of series of sections of frog embryos has 

 shown that in all cases the ' segmentation cavity ' is converted, 

 by fusion with the secondary or neurenteric enteron, into the 

 forward part of the enteron. 



Literature. 



1888, O. Schulze. — ' Zeit. f. wiss. Zool.', Bd. 47, p. 325. 



1891. Robinson and Asshetou. — ' Quart. Journ. Micr. Sci.', vol. xxxii, 



p. 451. 



1893. A. M. Marshall. — ' Vertebrate Embryology '. 



1900. H. V. Wilson. — ' Anatomischer Anz.', Bd. 18, p. 209. 



1902. A. Brachet. — ' Arch. d. Biol.', t. 19. 



1906. O. Hertwig. — ' Handbuch der Entwicklungslehre ', i. 



EXPLANATION OF PLATE 1. 



Figs. 1 and 2. — ^Horizontal sections of an early stage (Text-tig. 2). 



Fig. 3. — Sagittal section of typical stage indicating an early phase of the 

 invagmation of the dorsal lip (Text-fig. 3). 



Figs. 4-8. — Sagittal sections of a later stage, wherem the excavation 

 of the cells intervenmg between the primary and secondary cavities has 

 reached an advanced stage (Text-fig. 4). 



Figs. 9-14. — Transverse sections of a stage after fusion of the cavities 

 (Text-fig. 6). 



Figs. 15-18. — Transverse sections of a younger stage than the preceding 

 to illusti'ate the process of fusion (Text-fig. 5). 



