1904.] NATURAL SCIENCES OF PHILADELPHIA. 385 
the original archenteric invagination (5B, 5b, 4C, 5C, 5c, 4D, 5A) 
while dorsally and anteriorly are seen more yolk-ladened elements 
whosé origin may be traced to the large entoderm cell 4b? and those 
around it. The stomodzal invagination breaks through at a much later 
period between the descendants of 5a and 4b and their neighboring 
cells, which have been turned in an anterior direction, while doubtless 
cells from 4c and 5b also push in upon this region with the closure of 
the blastopore. By the torsion which the enteron has undergone the 
upper mass of large yolk-ladened cells is moved more and more to the 
left, while in like manner 4a? turns to the right. While this is occurring 
the invaginating shell-gland has pushed the anterior and posterior 
walls of the enteron very closely together, both enteric and cleavage 
cavities being practically obliterated (fig. 86). When this structure 
evaginates the enteron again opens out and has then lost its elongated 
form, being rounded with its wall cells in close contact (fig. 87). 
In Umbrella as well as in Fiona 4b? occupies the anterior end of the 
enteric mass pushing up into the pointed apex of the gastrula, and the 
same is true of Aplysia in which there are but two large blastomeres, 
though according to Blochmann’s nomenclature such does not appear 
to be the case. In later stages the positions of the large cells of the 
fourth quartet of Umbrella and Fiona are identical. The intestine of 
Umbrella is said to be formed by C” and D” (5c and 5d), which, as Hey- 
mons did not take into consideration an entoblastic contribution from 
4d, correspond fairly well to the conditions found in Fiona, where these 
cells he just at the place of origin of the intestine and may well take 
part in its future development. The cell-lineage of the archenteron 
of Crepidula is given as follows: “‘The four macromeres form the roof 
of the archenteric cavity. The cells of the fifth quartet form its lateral 
boundaries, arching the cavity on all sides save the posterior. Here 
the archenteric cavity runs backward between the cells 5C and 5D (5e 
and 5d) nearly to the posterior boundary of the egg. The cells of the 
fourth quartet come together on the ventral side of the archenteron, 
forming its floor anteriorly and ultimately giving rise to some of the 
many small cells which form that part of the mesenteron, adjoining 
the stomodeum.” ‘The intestine arises from the posterior lower right 
region of the enteron as a tube-like evagination, formed from the entero- 
blasts derived from 4d and neighboring small endodermal cells and 
ending blindly against the ectoderm. Later it elongates and the end 
is carried somewhat upward along the right side by trosion of the larva. 
It contains a lumen from the first. As the stomach begins to enlarge 
it is seen to be bounded by large cells dorsally and anteriorly in its lower 
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