384 PROCEEDINGS OF THE ACADEMY OF [April, 
of fig. 85. If we now examine section fig. 87, which is taken through 
a veliger slightly older than that shown in fig. 104, the relation of the 
enteron to its surrounding structures may be observed. The large 
entodermic cell, 4a”, has been successively traced through preceding 
stages from its origin on the left side of the archenteron to its final 
position on the right of the enteric cavity, as is shown in the figure. 
Just posterior to this will be noted a mass of cells connecting the enteron 
with the ectoderm. The nuclei of these cells are compact and deeply 
staining, and the cytoplasm is decidedly clearer and contains less 
yolk than that of the cells directly surrounding the enteric cavity. 
Moreover, their position beside the large cell 4a? and now, through the 
torsion which the enteron has undergone, their later position some- 
what posterior to this cell, indicates the probability of their correspond- 
ence with the ‘enteroblasts” of fig. 86 (Pl. XX XI) and earlier stages, 
in which the identity of these cells is unquestioned. 
It is proper in this place to consider again the results of Carazzi’s 
work on Aplysia and its relation to the mesentodermal history of 
Fiona. It will be remembered that Carazzi’s account of the lineage 
of 4d up to a stage when its derivatives number twelve cells exactly 
parallels my results on Fiona, but regarding the fate of these cells 
there is lack of agreement. The anterior small cells of Aplysia are 
believed to be purely mesoblastic, while at least four of them in Fiona 
appear, from the preceding account, to be entodermal in nature. 
Carazzi, however, derives endoderm from the two small posteriorly 
directed cells (e, e* of Aplysia) which correspond to z}, 2 of Fiona. 
These latter cells were last seen lying at the posterior end of the gas- 
trula of Fiona closely pressed against the ectoderm. Ata later period, 
when a large number of mesodermal elements lie in this region, the z?, 
z cells become indistinguishable from these. Sections of later stages 
(fig. 87) show two cells which are larger and clearer than the entero- 
blasts and which lie against the ectoderm where the intestinal mass 
touches it. They may represent the cells z', z?, but of this there is 
no evidence except that given above. Anal cells are not a marked 
feature of the developing embryo of Fiona, but at this time sections 
in particular show two cells of somewhat larger size than the surround- 
ing ectodermal elements, against which the forming intestine abuts 
and which are doubtless comparable to the anal cells of other forms 
(fig. 87, An.C). 
It will now be seen that the portion of the enteron lying most 
posterior and close against the shell-gland invagination has been 
derived from the cells which formed the bottom and the left side of 
