492 BERTRAM G. SMITH 



extent about the animal pole as a center, covering an area about 

 140 degrees in diameter. As gastrulation advances this clear 

 area becomes encroached upon at its posterior margin (figs. 122 

 and 123) by the extension of the opaque material. Meanwhile 

 the boundary of the roof of the blastocoele becomes more sharply 

 defined; before the upgrowth of the postero-dorsal opaque region 

 has reached the animal pole the margin of the blastocoele roof 

 is usually bounded by a sharply defined furrow, the 'septal fur- 

 row' of Ishikawa (see below) — a characteristic and almost unique 

 feature of the gastrulation of Cryptobranchus. The precise 

 stage at which this furrow appears varies considerably in different 

 eggs; figure 116 shows a case of unusually early appearance, 

 figures 117, 125 and 126 a stage in which it is usually well estab- 

 lished. Moreover, the distinctness of this groove varies greatly, 

 particularly in eggs of different spawnings; in some lots of eggs 

 the groove is established early and is very sharply marked, while 

 in occasional lots of eggs it is almost absent. 



The septal furrow appears first at the posterior margin of the 

 roof of the segmentation cavity, then extends gradually around 

 to its anterior margin; in its appearance and manner of extension 



Figs. 122 to 133 Stage 11 (gastrula) of Cryptobranchus allegheniensis. Free- 

 hand drawings of the living eggs, viewed by both transmitted and reflected light; 

 the proportions of the various parts are checked by comparison with camera draw- 

 ings of preserved material. The drawings are oriented with respect to the 

 vertical axis determined by gravity. The roof of the segmentation cavity is 

 nearly transparent; the roof of the gastrocoele is quite translucent, or slightly 

 opaque in the regions containing mesoderm; heavily yolk-laden regions are 

 decidedly opaque; bci, roof of blastocoele; blp., dorsal lip of the blastopore; /., 

 fenestra (roof of the blastocoele differentiated into a roof-like structure); gc, 

 gastrocoele; m., region containing mesoderm. X 5. 



Figs, 122 to 124 Upper hemisphere, lateral view and lower hemisphere of an 

 egg in the beginning gastrula stage. 



Figs. 125 and 126 Upper hemisphere and lateral view of an egg a little later 

 than the preceding. 



Figs. 127 to 129 Postero-dorsal view, upper hemisphere and lateral view of 

 an egg slightly later than the preceding. 



Fig. 130 Upper hemisphere of a slightly later egg. 



Figs. 130 to 133 Upper hemisphere, postero-dorsal view and lower hemisphere 

 of an egg near the close of Stage 11 (shortly before the appearance of the neural 

 groove). 



