DEVELOPMENT OF BOLOCERA 581 
involuted giving rise to the rudiment of the stomodoeal 
canal. 
The invaginating area soon presses against the trophenchyme, 
and we often find at this stage secondary flattening of the whole 
egg and foldings of its walls, which are probably caused by 
the resistance of the trophenchyme to the progress of invagina- 
tion. However, in course of time, the trophenchyme finds its 
way through the inpushing endcderm into the cavity of the 
archenteron. First, the fluid and fine granules begin to get 
through, then the yolk-spheres, and lastly the clear spherules. 
The process appears to be mechanical in the sense that the 
trophic material passes through interstices between endoderm 
cells, and is not first swallowed or assimilated and then excreted 
into the archenteron. 
As gastrulation proceeds most of the trophenchymal nuclei 
disintegrate, but some pass with the trophenchyme into the 
archenteron and are absorbed later. 
It is of particular interest to note that in a few cases the 
end-result of gastrulation is attained by a process which may 
be described more accurately as unipolar immigration than as 
invagination. In such cases the cells over a relatively small 
area at one pole of the blastula begin to sink inwards through 
the trophenchyme, at the same time proliferating and spreading 
out so as almost to lose their continuity with one another. 
This process continues until having passed through the whole 
depth of the trophenchyme, they abut against the ectoderm 
where they soon form a continuous sheet of endoderm lining 
an archenteric cavity which now naturally contains all the 
trophenchyme. Sometimes the process is intermediate between 
that described above and open invagination. Similar differ- 
ences occur among the eggs of different Crustacea, but not so 
far as I know among the eggs of the same Crustacean species. 
We may put down the variations in Bolocera as probably due 
to differences in the character of the yolk, noting that those 
blastulae which showed the fewest foldings and the least 
deformation tend also to form their endoderm by unipolar 
immigration. 
