312 LANGENBECK. [VoL. XIV. 
I have seen I have found nothing to which I might compare 
these two cells. Dr. McMurrich, however, found two cells in 
the interior of an advanced isopod egg; but as they were seen 
only once, and no traces of disintegrating nuclei were found in 
the later stages, Dr. McMurrich supposed it to be an abnor- 
mality. It may be, however, as Dr. McMurrich suggested to 
me, that the two cells found in the interior of the isopod egg 
are comparable to those which I have described for Microdeu- 
topus. Dr. Conklin has kindly permitted me to state that he, 
too, finds that two cells (the tip cells in one arm of the cross) 
in Crepidula are lost in the later stages. However, they are 
not, as I understand, absorbed, but thrown out. It would be 
interesting to know if these cells in Crepidula could be com- 
pared to those in Microdeutopus. Comparisons have been made 
between the amphipod and molluscs before (Ulianin 'g1), with 
how much right is still to be decided. But certainly the loss 
at an early stage of two blastomeres is very remarkable, and 
has as yet not been described for other forms. 
44-cell stage. — The blastomeres £%, F* divide, giving rise 
to £*, F and E”, #2, The cleavage is parallel to the planes 
which divided £ and F before. Fig. 21 shows an egg passing 
from the 44-cell stage into the 46-cell stage. The blastomeres 
E~ and F” are dividing for the last time before they rise to the 
surface of the egg. 
73-cell stage. — Figs. 14-16 represent three views of an egg 
of seventy-three cells. In this ege 2°5 £21) hor) 8%, and 
F® have all been divided by vertical planes at right angles to 
the last, and the micromere f has also divided in the same way. 
The protoplasm in these cells has come up to the surface, and 
the nuclei have become very large and clear. This is the first 
appearance of the blastoderm. The sixteen large macromeres 
have divided into thirty-two by an equatorial cleavage. aa’, 
cc’, and 6’-6+, d*-d* have not changed, and there are seventeen 
cells derived from the gi and GH groups. The descendants 
of A and C lie next to each other and border upon the EF 
group, while the B and D groups are forced apart by the GH 
group. Sections (Fig. 26) show the protoplasm quite near the 
surface at this time, the cell boundaries breaking down at their 
