1904.] NATURAL SCIENCES OF PHILADELPHIA. 393 
blast. The mesoblast arises in part from the fourth quartet cell 
of D quadrant. The remaining fourth quartet cells and all the 
macromeres are entoblastic, as is also the case with a small portion 
of 4d. 
The first quartet of ectomeres give rise to the trochoblasts and ecto- 
blastic cross. To the latter structure are added as “tips” the upper 
cells of the second quartet in all quadrants. The cross is radially 
spiral in symmetry, and does not increase in breadth by transverse 
splitting of its arms until a comparatively late period. Cells from the 
first quartet form the head vesicle, cerebral ganglia and eyes, and a 
portion of the first velar row. 
The second quartet has a similar cleavage history in all four quad- 
rants until a stage of about 150 cells. In later development the 
elements of this quartet in D (posterior) quadrant show great 
inerease in size and divisional activity, initiating the posterior point 
of growth, with resulting bending of the embryonic axis. Cells from 
this area form the shell-gland and median portion of the foot. A 
large number of second quartet cells from the anterior and lateral 
groups aid in the formation of the velum. The more ventral elements 
of B quadrant help to close the blastopore. 
In the third quartet bilateral cleavages first appear in the posterior 
quadrants (cells 3c! and 3d‘). Secondary mesoblasts arise from the 
anterior quadrant groups of this quartet (cells 3a", 3a” and 3b™, 
3b”1), The large anal excretory cell (3c) and its associated cells 
are derived from C quadrant of this quartet. Third quartet cells 
surround the blastopore as it closes, with the exception of a small 
anterior portion; much of the stomodeum and the lateral portions 
of the foot come from third quartet elements. 
The mesoblast of Fiona is derived from two sources, ento-mesoblast 
from 4d and ecto-mesoblast from the third quartet in A and B quad- 
rants. The greater amount comes from 4d and forms teloblastic 
bands in the posterior region of the gastrula. The secondary mesoblast 
(ecto-mesoblast) is largely “larval” in fate, since much of it goes to 
form the muscles of the velum. From the history of 4d it appears 
that this cell contains both mesoblastic and entoblastic derivatives, 
the latter taking part in the formation of the intestine. 
‘As is the case with many Opisthobranchs, the gastrula is sharply 
pointed anteriorly, the apical point at first lying at the end of the 
anterior arm of the cross. 
The blastopore at the time of closure is surrounded by third 
quartet cells, except at its anterior edge, where second quartet cells are 
