480 A Sketch of Comparative Embryology. [July, 
proved extremely fruitful, as others have been useless or mislead- 
ing. Therefore, in spite of Haeckel’s great and unusual endow- 
ments, which every one must recognize and admire, it is unsafe to 
quote his writings as authorities in matters of fact! Having 
given my own opinion, I may add that while many of the younger 
naturalists bestow an almost unqualified admiration on Haeckel, 
several distinguished zodlogists severely condemn him as unsci- 
entific. 
In order to understand the embryology of sponges, it is neces- 
sary to consider briefly their structure. The sponges of com- 
merce are merely the skeletons of the living animals, the soft 
portions having been removed by maceration. During life the 
fibres, which make up the skeleton, are all covered by cells. The 
mass of the sponge is permeated by intercommunicating canals, 
connected with the exterior by numerous openings upon the sur- 
face, these openings are of two kinds, smaller ones called pores, 
by which currents of water enter the canals or tubes, and larger 
ones, or in some cases a single orifice, the osculum, through which 
the water passes out. The entire surface of the canals is lined by 
a continuous layer of cells, the extoderm. Over definite areas of 
this lining the cells are cylindrical, have a so-called collar, and 
are provided, each, with a single long: sweeping cilium, or fagel- 
lum (geissel), while over the intervening parts, the lining is com- 
posed of simple flat polygonal cells. In a few sponges ( Ascones) 
the whole canal system is carpeted by flagellate cells. The 
flagella maintain the currents of water, sweeping in the particles 
of food, which are seized by the sponge as the water runs 
through. The external surface is entirely covered by a continu- 
ous stratum of flat polygonal cells, the ectoderm, between which 
and the canals lies the thick middle layer or mesoderm, in which 
the skeleton and the sexual products are developed. 
The mesoderm is composed of numerous independent cells, 
each separated from its neighbors by amorphous intercellular sub- 
stance, the specific character of which varies from species to 
species. Its consistency may be so slight that the cells can crawl 
about through it, like Amcebas. A certain portion of these cells 
are transformed into the genoblasts; usually either only eggs or 
only spermatozoa are produced ina single individual, but of those 
sponges, whose sexuality is known, a few are hermaphrodite. 
1In Huxley's Anatomy of Invertebrates, the chapter on sponges is based on 
Haeckel’s work and contains several important errors. 
