Hawaiian Marine Gastropods — OSTERGAARD 
87 
Fig. 8. Mitra astricta Reeve, a, Egg capsules 
attached to rock; b, enlarged capsule; c-g, cleavage 
stages; h, blastula stage; i, gastrula stage; j, embryo 
showing rudiment of shell; k, veliger; l, m, free- 
swimming veliger: l, ventral, and m, dorsal as- 
pects; tir-p, shell of free-swimming veliger larva. 
of the blastomeres after the first and second 
divisions seems a usual condition in the gas- 
tropods, but it was more pronounced in this 
case than generally. Cleavage proceeded ra 
ther slowly at first, the four-cell stage having 
been reached only after 24 hours; the follow- 
ing 24 hours presented a stage in which 20 
or more micromeres capped the animal pole. 
There were four macromeres (Fig. 8f, g ) . 
After 3 days of development the micro- 
meres had greatly increased in number and 
constituted about one-half of the surface area 
of the embryo; a number of those near the 
vegetal pole had developed short cilia, while 
the four large macromeres were as yet un- 
divided. Projecting out from each side of the 
embryo was a large micromere, clear like the 
others, about half of its sphere protruding 
beyond the surface of the ectoderm. This 
stage was evidently comparable to the blas- 
tula, although the space required for the seg- 
mentation cavity was entirely obstructed by 
the large macromeres (Fig. 8 h). 
A day later — after 4 days’ development- — 
the embryo had become elongate and the 
micromeres had surrounded by a complete 
layer (ectoderm) the dark, conspicuous mac- 
romeres, which also had taken on an elon- 
gate form, but the boundaries of which had 
now become so obscure that it was impossible 
to determine their number. At the anterior 
end of the macromeres there was a clustei 
of rather large, clear cells, probably meso- 
dermal. Anteriorly the ectodermal layer, in 
connection, perhaps, with an underlying mes- 
oderm, had developed three slight lobes, the 
rudiments of the velum. Cilia could not be 
seen although they must have been present 
as the embryo showed considerable activity. 
This was the gastrula stage (Fig. 8/). 
After the sixth day a shallow cup-shaped 
shell surrounded the posterior region of the 
embryo (Fig. 8 j). Seven days of develop- 
ment brought out the veliger stage. A bi- 
lobed ciliated velum which had minute eye- 
spots was found. A short pyramidal foot was 
