1904. ] NATURAL SCIENCES OF PHILADELPHIA. 391 
CHANGE oF AXIS AND FORM OF THE DEVELOPING ORGANISM. 
The ege at the time of laying is spherical. With the division into 
four cells the primary egg axis, running between the centers of the 
animal and vegetative poles, becomes shorter than the diameter of the 
equatorial plane. As segmentation proceeds this relation persists 
(fig. 14), and with continued division the formation of a large cleavage 
cavity becomes more pronounced. Until the cleaving egg reaches a 
stage of over sixty cells its surface, when viewed from either pole, 
appears almost perfectly rounded, but shortly after this its antero- 
posterior axis becomes shorter than the lateral (figs. 45, 56, 74), this 
relation holding until increased growth in the posterior and anterior 
quadrants causes elongation in that direction. Until about a stage 
shown in fig. 74 the primary egg axis, running from the center of the 
animal to the center of the vegetative pole, follows a straight line. 
Immediately after this, accentuated growth of the posterior region 
initiates a bending of this axis, which finally results in its complete 
folding upon itself, or a rotation through 180 degrees. A sharply 
pointed anterior projection arises (fig. 78), while at the same time the 
posterior dorsal region is rapidly increasing in extent and changing 
the embryonic axis. As the gastrula elongates the apical pole is moved 
forward, and by the time the first velar row becomes distinct the origi- 
nal polar axis has become so bent upon itself as to form an angle of 
nearly 90 degrees (figs. 95, 98). With the continued multiplication 
of cells in the head region that portion of the larva changes from its 
originally pointed shape into a rounded though not prominent head 
vesicle, while at the same time the opposite end is rounded by continued 
growth of second and third quartet elements (figs. 100,101). The orig- 
inal polar axis will be seen in these figures to have moved through about 
135 degrees. In the next stage, represented by figs. 102 and 104, 
the head vesicle has reached its largest relative size when taken in 
connection with the veliger as a whole. Comparing these figures with 
those which have. gone before, a marked increase will be seen in the 
antero-posterior depth, and if this be considered in connection with the 
ereat change of axis the enormous growth of the posterior region will 
be evident. It is generally conceded that the head vesicle of mol- 
luscan and annelidan larve is of functional importance in serving as a 
float. In Fiona the head vesicle is never large and prominent and a 
substitute may reasonably be expected. With the differentiation of 
the velar lobes and foot the shell-gland may in figs. 105, 106 and 107 
