1904.] NATURAL SCIENCES OF PHILADELPHIA. 341 
Umbrella, although Heymons’ conclusion regarding the fate of the 
descendants of 4d is at wide variance with the conditions which are 
found in Fiona. After the cleavage of 4d into equal parts, Heymons 
states that two small cells are given off from these, so that they lie in 
the posterior region of the macromeres. It is very evident from his 
figures that these cells, which would correspond to E?, E? of Fiona, 
at first lie quite dorsal to the enteron and in the cleavage cavity. The 
large cells next divide nearly equally, the most posterior being slightly 
smaller and corresponding in size and origin to m’z’, m’z?._ These 
latter shortly change their position in Umbrella exactly as in Fiona, 
for, says Heymons, “Bald beginnt eine interessante Lagerungsver- 
schiebung einzutreten. Es rucken namlich die hinteren Zellen weiter 
nach dem animalen Pol hin und legen sic vollkommen auf die vorderen 
auf”. While this rearrangement is occurring and after its completion 
two and later other small cells are given off by the large underlying 
cells toward the smaller cells originally budded forth. Exactly the 
Same process occurs in Fiona—compare Heymons’ figs. 23 and 24 with 
my fig. 71. Heymons’ smaller cells M’, M’ (corresponding to miz!, 
m’z? of Fiona), which have moved toward the animal pole of Umbrella, 
do not appear from the account to divide again so quickly as 
in Fiona, but that they later divide teloblastically is evident. 
As has been mentioned before, the small anterior cells of Umbrella, 
which correspond to E!, E?, e!, e?, of Fiona, at first lie entirely within 
the segmentation cavity. Figures of later stages, however (Heymons’ 
fig. 29), show that they then lie at a level with the posterior cells of the 
enteron (D, A’, C’, etc.), and are directly between these and the anal 
cells. The same relative position is taken by the corresponding cells 
of Fiona. 
In interpreting the results of Heymons the above point of view is 
somewhat different from the comparison of Conklin between Umbrella 
and Crepidula, in which he suggests a resemblance and possible simi- 
larity of origin between the enteroblasts of Crepidula and the telo- 
blastic cells M, M, M’, M’, of Umbrella. In both these “are large cells 
containing a considerable quantity of yolk, about equal in size and 
grouped in a characteristic way”; but the same may be said of the 
similar cells of Fiona, yet they have no part whatever in the formation 
of the enteron, though from their appearance I was led to think such 
might be the case before a knowledge of their later history proved 
otherwise. The explanation of the whole matter lies in the axial 
change which the derivatives of 4d have undergone in the forms con- 
sidered. The posterior macromeres (particularly D) of Umbrella are 
