1880.] - A Sketch of Comparative Embryology. 483 
by invagination. The skeleton begins to appear about the time 
the larva fixes itself. 
In the other type of development, which has been observed in 
the higher calcareous sponges, there is both a temporary pseudo- 
gastrula, and a permanent gastrula differently formed, which is 
directly metamorphosed into the permanent sponge. The pseudo- 
gastrula normally occurs only before the larva leaves the follicle 
of the parent body, and arises by the turning in of the large cells, 
just as the finger of a glove may be inverted; the larva then 
appears like a cup formed of two membranes, the outer of small 
cells, the inner of large. Before long, however, the large cells 
are everted, and the embryo (Fig. 16) reassumes the characteristic 
egg-shape, and soon leaves the parent, swims about freely for two 
or three days, and finally perma- 
nently attaches itself. 
While still free, it broadens, and 
its long axis shortens (Fig. 17), 
whereby the large cells begin to 
grow over the small ones, which 
are gradually pushed in more and 
more until they are fairly invagi- 
nated. The large cells advance Fic. 17.—Older stage of Fig. 16. 
further, gradually constricting the opening until it becomes quite 
small. Viewed from the oral side, at this stage, the embryo pre- 
sents the appearance indicated by the outline, Fig. 18, 4.2 At 
this stage the larva fixes itself by its oral end. The cells around 
Fic. 18.4, Oldest free-swimming sta: f oral surface. B. The same after 
i ge, view of ora 
attachment. Larva of adarei after F. E. Schulze, magnified about 240 diam 
sd mouth nestle against the underlying surface, and send out 
+ ag fon none upper and inner parts-show through, rendering the outlines much 
ss 
