EMBRYOLOGY OF CRYPTOBRANCHUS 495 



Later changes are concerned with the forward, or rather ven- 

 trad, progress of the septum and the increase in the extent of 

 the translucent roof of the gastrocoele, with a correlated ventrad 

 movement of the fenestra and a diminution of its area; there 

 is a slight increase in the extent and opacity of the mesoderm 

 (figs. 118 to 120 and 130 to 132). The fenestra finally closes 

 just below the horizontal equator on the ventral side of the egg 

 (fig. 121). The changes in the position and extent of the fenestra 

 are shown diagrammatically in figures 134 to 137. 



The foregoing detailed account of the progress of the septum 

 as viewed from the exterior in the living egg of Cryptobranchus 

 clears up whatever doubt may exist as to the significance of the 

 'shadowy area' described in the gastrula of Spelerpes by Goodale 

 ('11) and confirms his suggestion as to the nature of this area. 



Ishikawa ('08 and '09) describes in the early gastrula of Cryp- 

 tobranchus japonicus a furrow bounding the roof of the blasto- 

 coele at its posterior margin, which he calls the 'Scheidewand- 

 furche' or 'septal furrow.' As compared with the furrow of 

 similar nature described above for C. allegheniensis, it is earlier 

 in making its appearance, since it antedates the blastopore. 

 The area later enclosed by this furrow has been named by Ishikawa 

 the 'Keimhohlensegment' or 'blastocoele-segment' ; judging from 

 his figures its later history is much the same as that of the cor- 

 responding structure, which I have preferred to call the 'fenestra,' 

 in Cryptobranchus allegheniensis. 



The only mention of similar structures which I can find in the 

 literature on other forms is a description by Hatta ('07) of a 

 groove which he calls the 'boundary groove' in the gastrula of 

 Petromyzon. As compared with the septal furrow of Crypto- 

 branchus this groove is greatly exaggerated in Petromyzon, con- 

 stricting the egg so that in some cases it assumes an hour-glass 

 form. 



As suggested by Hatta, the boundary groove or septal furrow 

 is passive in origin, and a product of gastrulation. Similar con- 

 ditions have produced it in two such widely separated forms as 

 Cryptobranchus and Petromyzon; in each case the egg contains 

 considerable yolk, and the roof of the blastocoele is unusually 



