1004.] NATURAL SCIEXCES OF PHILADELPHIA. 391 



Change of Axis and Form of the Developing Organism, 



The egg at the time of laj'hig is spherical. ^Yith 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 wall 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 ^\ith the 

 great 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 larvse 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 



