1904.] NATURAL SCIENCES OF PHILADELPHIA. 345 
to form the enteron has already begun by the depression of the smaller 
cells which lie in the center of the vegetative pole, while the small cells, 
E', E?, e', e?,.at the anterior end of the teloblasts have become drawn 
into the posterior region of the invagination (except for some varia- 
tion, an instance of which is shown in fig. 72), where at this time they 
help to close that portion of the gastral pit. As the primary enteric 
cells sink into the cleavage cavity the small cells, E!, E’, e!, e?, come 
into close connection with the posterior edges of 5C, 5D,4a. Thus a 
more or less complete cup-like invagination is brought about among 
the entomeres, in which the smaller elements lie at the bottom with 
the larger (4a?, 4b”, 4c?) between, and the small cells which have arisen 
from these latter lying peripheral to them. Above, toward the ven- 
tral surface, lie small cells of the second and third quartets around the 
blastopore opening. 
In the formation of the enteric cells the manner in which the fourth 
quartet arises appears to be characteristic of a number of Opistho- 
branchs. This quartet is in Umbrella (Heymons, 1893), Aplysia 
(Blochmann, 1883; Carazzi, 1900) and Tethys (Viguier, 1898), as well 
as in Fiona, larger than the macromeres remaining at the center of 
the vegetative pole. 
The further development of the enteron will be discussed later. 
CLEAVAGE HISTORY OF THE ECTOMERES. 
As has been seen, the ectoblast arises immediately after the four- 
cell stage by the three successively alternating cleavages in which the 
macromeres participate, giving rise respectively to the First, Second 
and Third Quartets of micromeres. The cleavage history of these 
cells will now be taken up and their ultimate fate, as far as can be 
determined, considered. 
The First Quartet. 
The formation of the ‘“‘turrets,” 1a?-1d?, and the “‘apicals,”’ la‘-1d!, 
leading to the radially symmetrical twenty-four-cell stage, has already 
been considered. Shortly afterward the apical cells divide in a dexio- 
tropic direction, thus alternating with the preceding cleavage, and by 
this division the four “‘basal”’ cells of the ectoblastic cross arise, while 
between these and the central point of the egg lie the four small apical 
cells from which they were derived (fig. 23). Before this cleavage had 
occurred the upper and dextral cells of the second quartet had in each 
quadrant given off a small cell in a leotropiec direction (fig. 21), which 
