DEVELOPMENT OF SPELERPES BILINEATUS . 195 



the presence of the groove, but do not attempt to assign it any 

 significance. They consider that the very short line of real 

 fusion of the blastopore lips, which takes place after the blastopore 

 has become a vertical slit, is the primitive streak, since here 

 all three primary germ layers are fused. Altogether there is 

 much confusion on the subject, but I believe we may safely 

 dismiss the view of the first two groups of authors as untenable; 

 that of the first because of the lack of proof of concrescence and 

 that of the second because there is no fusion of the three germ 

 layers along the median groove, which fusion is now the generally 

 accepted criterion of the presence of a primitive streak. 



Gastrulation 



Just before the egg is ready to gastrulate, a vertical section 

 (cf. fig. 28, which represents a slightly later stage) shows a well- 

 developed blastocoele, be, whose nearly level floor passes without 

 break into the side walls. Its roof at the upper pole contains 

 one or two layers of cells with small granules. It thickens down- 

 wards to three or four layers of large cells whose granules are of 

 intermediate size. The yolk cells are polygonal, except where 

 rounded next the blastocoele. 



A vertical section of an early gastrula is shown at fig. 28; the 

 corresponding external view in fig. 22. The blastopore, b, a narrow 

 groove (cleft or split) lies about 30° below the equator among the 

 yolk cells, which are triangular at the bottom of the groove. 

 They appear to be pulling away from the surface as has been noted 

 in other Amphibians. 



A little later (fig. 29, cf. fig. 23), there are numerous triangular 

 cells about the blastopore whose long axes are all directed dorso- 

 ventrally. In the living egg they are flask- or bottle-shaped 

 with very long necks. The floor cells of the blastocoele are now 

 separating from one another, while the floor itself is arched. 

 Over the blastopore, it is raised into a tongue, /, leaving a cleft, 

 c, between it and the outer cells as noted by H. V. Wilson and 

 others. 



