518 BERTRAM G. SMITH 



In these later stages, the blastopore is doubtless often indistin- 

 guishable in living material (figs. 274 to 276). 



A triradiate form of the blastopore is not so frequently found 

 in Neeturus; in Cryptobranchus japonicus (Ishikawa '08), it often 

 occurs. A general resemblance may be noted between the blas- 

 topore of the urodeles cited and that of the dipnoans (Ceratodus, 

 Semon '01 ; Protopterus and Lepidosiren, Kerr '09). 



B. Summary 



Gastrulation involves a combination of the processes of invag- 

 ination or emboly, and overgrowth or epiboly. 



During gastrulation the roof of the segmentation cavity be- 

 comes very thin, and is bounded superficially by a sharp furrow, 

 the 'septal furrow' of Ishikawa. 



On account of the translucent character of certain parts of 

 the egg, many of the internal changes concerned with gastrulation 

 can be followed quite satisfactorily in Uving material. 



For some time after the beginning of gastrulation, the vegetal 

 pole may be located through the intersection of the first and 

 second cleavage furrows. 



During gastrulation and the formation of the neural groove 

 and neural folds the egg rotates on an axis at right angles to the 

 median plane so as to bring the morphological axis at an angle 

 of 56 degrees from the vertical. 



The dorsal lip of the blastopore is formed about 68 degrees 

 above the vegetal pole; the ventral lip is formed much later at 

 an equal distance on the other side of the vegetal pole. Since 

 the closing blastopore lies approximately at the vegetal pole, 

 overgrowth proceeds through equal distances on the dorsal and 

 the ventral sides of the egg. During the early part of gastrula- 

 tion, before the ventral lip is formed, overgrowth takes place 

 rapidly and extensively at the dorsal lip; after the blastopore 

 has become a complete circle, overgrowth takes place very slowly 

 at the dorsal lip, very rapidly at the ventral lip. 

 . Not until after the neural folds are well formed is the yolk 

 plug completely overgrown; as compared with Neeturus and 



