ON tin: UASTRULATION IX TETROMYZON. 5 



grooved part marked witli A {Figs. Aa and Ah), so tliat the oviiiu 

 appears as if it were truncatud at thi.s i)art, as may ho well seen 

 in a lateral view. The field tliat lies between the groove and 

 the flattened area of the base is gradually produced into an 

 eminence which is soon converted into a cone witli rounded i\\) 

 (Fig. 4a, c. em). The cone is directed not merely outwards, but 

 also forwards, in extreme cases overhanging the groove in front 

 like a beak, while laterally it slopes downwards to the right and 

 left like a pair of wings {Fig. 45). 



Corresponding to the changes which have just been mentioned, 

 the view from the lower pole of the ovum necessarily undergoes 

 notable alterations. It is no longer circular in outline, but is 

 pear-shaped being gradually narrowed towards the protruded 

 (dorsal) part and coming to a point in that part as if it had been 

 compressed from side to side, while the remaining (ventral) part 

 is still rounded as before. The ventral rounded part still retains 

 a spherical form, while the compressed part is, as just noticed, 

 flattened. This flattened part of the basal surface remains, how- 

 ever, not long in this condition, but is gradually transformed 

 into a slit-like depression {Figs. 5a and 5b, bp.) ; it is this de- 

 pression which w^as spoken above as an early trace of the 

 blastopore. When the latter becomes obvious, the conical eminence 

 grows for a time still more j)rominent {Fig. 5a), but subsequently 

 it becomes depressed assuming a flattened out appearance 

 {Fig. 6a). 



According to my view, this series of occurrences is nothing else 

 than the beginning of the gastrula invagination. As the lower opaque 

 hemisphere begins to invaginate within the upper hemisphere, {. e., 

 to push on toward the op2:>osite pole, it at first flattens instead of 

 remaining convex as heretofore. The pushing inward is, however. 



