482 A Sketch of Comparative Embryology. [July, 
bursting the walls of the follicle, passing into the canal system, 
and escaping through one of the pores. At the time of its birth, 
the larval sponge has very distinctive peculiarities, and differs 
strikingly from all other larve. 
The larva, when hatched, is egg-shaped (Fig. 16), the larger end 
is composed of large ceils with granular 
contents, which hide the nuclei, while 
the pointed end consists of small cells, 
each of which bears a long vibratile hair, 
or flagellum. It is by these that the 
larva swims. During segmentation, 
however, the cells are all more or less 
alike, and the differentiation takes place 
in some species earlier, in others later, 
so that in some sponges (Halisarca), 
——Egg-shaped larva, there is even a stage in which the whole 
Fic 
Toone ea yimming stage. 
Sycandra raphanus, after F.E. surface of the larva consists of small 
Schulze ut 530 diam 
cells, and later, those cells around the 
large pole of the egg grow bigger and granular. Again, in some 
_ forms (e. g. Chalinula) the difference between the two sets of cells 
is much less, and the small cells cover a proportionately much 
larger area than in the embryo figured (Fig. 16). 
There are also cells in the interior of the embryo, leaving, 
however, in certain cases a central cavity. Schulze states that in 
Sycandra there are no central cells, but Metschnikoff describes 
and figures them. These central cells are regarded by several 
authors as the primitive mesoderm. 
The metamorphosis of the larva into the sponge has been 
observed in but very few species. The change takes place 
according to two distinct types, which cannot at present be 
brought into relation with one another, because in the first 
( Sycandra ), the large cells form the ectoderm, and the small cells 
the entoderm, while in the second (Chalinula and Halisarca), the 
destiny of the two sets is exactly reversed, the small ciliated cells 
remaining external, the large cells becoming internal. In the 
latter case the embryo attaches itself by its broad end to a solid 
body, the small cells grow over the whole of the exposed surface ; 
a branching cavity is formed in the interior, and pores and an 
osculum break through. There cannot be saiď to be any gastrula 
stage at all, nor does the osculum answer to an opening formed 
