ON THE DIl'LOLiHOKDA. 387 



extremely rapid if one may judge by the small proportionate 

 number of embryos to be found at these stages, and the very 

 small extent of the coils which is occupied by embryos at this 

 stage. The spherical blastula appears to become more or less 

 hemispherical by a complete invagination of the lower and 

 rather larger cells, so that the gastrulation is typically embolic. 

 Fig. 11 shows the view from below of an embryo in which 

 the invagination is nearly completed ; fig. 6 is a lateral view 

 of the same embryo. The blastopore at this stage is large and 

 circular, with a diameter little less than that of the whole 

 blastula. In the next stage which I have found (fig. 7) the 

 embryo has commenced to elongate slightly in an axis 

 perpendicular to the principal axis of the blastula, and corre- 

 sponding very nearly with the long axis of the late larva. 

 Hence in ventral view (fig. 12) the embryo appears to be oval 

 in outline, and the blastopore is correspondingly oval. At the 

 part which we may now distinguish as the posterior end, the 

 oval blastopore is seen to taper off into a groove which extends 

 as a furrow vei'y nearly to the posterior border of the embryo. 

 These appearances certainly lead one to the inference that 

 the blastopore is closing up b}^ an approximation of its lips 

 in the posterior region. 



A reference to the lateral view of the same embryo (fig. 7) 

 shows that the anterior part of the embryo, mainly in front 

 of the blastopore, is increasing in size rapidly, and is tending 

 to bend over ventrally. 



At the next stage (fig, 13) this increase of the anterior 

 region has caused the outline of the embryo as seen from 

 below to apparentl}^ taper at the posterior end. The blasto- 

 pore has now further contracted in extent, and narrows off 

 posteriorly in connection with the median groove, which 

 has increased in length. In comparing this stage with the 

 last we note that the anterior lip of the blastopore appears to 

 be further back than in the latter, which is probably due to 

 the growth of the anterior region and to its further ventral 

 flexion (cf. fig. 8). On the other hand, the appearances here 

 still further confirm the result arrived at above, that the 



