ON THE GASTRULATION IN PETllOMYZON. 11 



nation is carried on. Tii the Anura, on the otlier hand, the yolk- 

 hemisphere shows no indication of a depression on its surface at all 

 and is exposed on the surface of the ovum down to a late stage until 

 the gastrulation is completed ; this exposed part of the yolk-field 

 is what is called the yolk-plug. It is tliercfore plain that the 

 invagination of the yolk-hemisphere is, in this case, not carried 

 on by its own power alone, but is to a large extent eftected 

 by tlie sinking sickle-furrow, that is, by the infolding animal 

 layer : in slioi't, its own activity is almost nil. To this point, I 

 will return further on. 



In the second place, no less striking are the changes under- 

 gone by the boundary groove during the gastrulation. It was 

 mentioned above that this groove becomes obvious for the first 

 time at a certain point on the junction-line and gradually 

 embraces the ovum {Figs. oa-4J), hg.), constricting it into two 

 unequal portions : the larger translucent and the smaller 02:)aque. 

 For a certain interval of time it does not, however, surround the 

 ovum completely {Figs. ba-Qb) ; that part of the ovum just op- 

 posite the point of its first appearance remaining free from the 

 constriction, although this part becomes likewise involved eventu- 

 ally {Fig. la). With the increasing growth of the conical 

 eminence which, it will be remembered, appears below the boundary 

 groove, the latter not only cuts in deeper, but is pushed 

 forwards by the eminence to cause in its middle portion a 

 curvature with the convexity turned toward the front : in 

 other words, the boundary groove retains no longer its original 

 transverse position. Tow^ards the ventral part, the groove becomes 

 shallow passing over into the junction-line which in this 

 part still remains in the original condition {Figs. 4ia and 4b, 

 bg.). As the visible blastoj^ore appears and consequently the 



