340 PROCEEDINGS OF THE ACADEMY OF [April, 
blasts in Crepidula and Fiona is the direct result of epibolic gastrulation 
in the one case, embolic in the other, which is in turn caused by the 
quantity and nature of the yolk which the 
macromeres contain. An intermediate 
condition is found in Nereis (Wilson, 
1898). Text-figure 1 (a) shows a sagittal 
section through the cleaving egg of Crepi- 
dula after one enteroblast has been sepa- 
rated from the mesoblast. The ectoblast 
has here but half covered the yolk, and 
the entoblastic element is thrown down- 
ward and backward in the direction in 
which it must go if it follows the ecto- 
derm over the yolk, and finally reaches a 
position posterior to the blastopore as 
that structure is closing (Conklin’s fig. 
61). In Nereis, text-figure 1 (b), the ec- 
toderm has advanced much farther over 
the yolk when the enteroblasts arise, and 
here we see that these elements are also 
directed downward but at the same time 
anteriorly. The next and last step in 
their change of position is illustrated by 
Fiona, text-figure 1 (c), im which, on ac- 
count of its invaginate gastrula, the en- 
teroblasts are not only anteriorly directed, 
but also at first le higher than the cells 
ig. «.1-= Bagibll ecohons from which they arose. 
through the gastrule of In Trochus (Robert, 1903) the meso- 
(a) Crepidula (Conklin), hjast arises at about the sixty-four-cell 
(b) Nereis (Wilson) and ‘ ae ; 
(c) Fiona. The entero- Stage by a lxotropic division which sepa- 
blasts are lined, the meso- yates the very large cell 4d from 4D. 
blastic colle Rumeied This cell divides dexiotropically and 
equally when eighty-nine cells are present. When there are one hun- 
dred and eighteen cells, each of the two derivatives of 4d divides, and 
of the resulting four cells the anterior pair are the smaller. Later 
the two larger posterior cells divide. Robert has not found endo- 
dermal elements to arise from 4d,but does not reject the possibility 
of such a condition. 
As might be expected from their close relationship, a nearer corre- 
spondence in the cleavage series is found when we compare Fiona with 

