340 
SAMUEL RITTENHOUSE 
remains slightly larger than the posterior, but the difference is not 
as great as in the blastula. During the blastula stage the larva 
swam near the bottom of the dish; when it attains the planula 
stage it rises and swims at or near the surface of the water for a 
shorter or longer time. This phenomenon occurs about twenty- 
four hours after the egg is fertilized. After several hours the 
planula gradually settles toward the bottom again and finally 
the spiral movements cease, due to the loss of the cilia. For a 
time of varying length after the spiral motion stops, the planula 
ghdes along the bottom of the aquarium. About forty-eight 
hours after the eggs are laid the larva reaches the stage of develop- 
ment in which attachment takes place. In preparation for attach- 
ment the planula settles to the bottom, loses its cilia and ceases 
its movements. 
FORMATION OF THE ECTODERM 
The formation of the ectoderm in Stomotoca apicata is simple 
in comparison with that of those species in which the segmenta- 
tion of the egg is unequal, giving rise to macromeres and micro- 
meres ; and in which the ectoderm is formed by a rapid increase of 
the micromeres and overgrowing of the macromeres by the proc- 
ess of epibole In Stomotoca, on the other hand, the cleavage 
is equal and at the completion of segmentation the blastomeres 
have divided into cells of uniform size and are situated in a single 
epithelial layer around the periphery of the blastula (figs. 16 and 17 
show sections of blastulae five and eight and one-half hours old 
respectively). Thus, from their position, all the cells which 
directly result from the segmentation of the egg may properly 
be regarded as forming ectoderm; and indeed might already at 
this stage of development be designated as such, were it proper 
to use the term ectoderm before the appearance of the inner germ- 
layer. The cells of the blastoshere are columnar in shape and, 
at first, all are comparatively of the same height; but finally 
those cells at the posterior end become somewhat taller than the 
rest. This is the region where the entoderm will be budded off. 
