1880. ] A Sketch of Comparative Embryology. 481 
The various kinds of sponges are distinguished principally by 
their external shape, and the peculiarities of their skeleton and 
canal system. The form from which all sponges may be deduced 
is the Olynthus type, which has the following characteristics: 1, 
it is attached by its base; 2, there is a large vertical central cavity, 
which, 3, communicates with the exterior at the upper end, 
through the osculum, and 4, at the sides through the secondary 
canals and pores. Modifications, besides those before mentioned, 
occur in the relative size of the main cavity, and by the formation 
of additional oscula. 
The principal kinds of sponges may be tabulated as follows: 
A WVU RY eUn Cy vac cu Gi soca cc ce bak bs wa Myxospongiz. 
B. With horny fibers (bathing sponges)............./...Spongide. 
C. With siliceous spicules (several distinct families)..siliceous sponges. 
D. With calcareous skeleton Calcispongiz. 
The Physemaria, which Haeckel described as multicellular 
organisms, representing a permanent adult sponge-like gastrula 
condition, have excited the greatest interest among zoologists. 
Recent investigations, however, render it probable that Haeckel’s 
description is entirely erroneous, and that these animals are really - 
multinucleolate Rhizopods. 
The gemmule, or winter buds, are not organs of sexual repro- 
duction, but rather of regeneration. ‘The tissues hibernate in a 
simplified condition, forming germ masses, the so-called buds; in 
the spring the sponge is regenerated by the renewal of its histo- 
logical differentiation. 
The formation of the egg presents no features requiring special 
comment from us. No polar globules have been discovered. 
Since the eggs and spermatozoa are ripe at the same time, the ova 
probably require to be fertilized, but I think no stage of the act 
of impregnation has yet been observed. The egg early becomes 
enclosed in a special capsule or follicle, developed by the neigh- 
boring cells of the mesoderm disposing themselves in a continu- 
ous layer around it, Within this follicle segmentation and the 
development of the embryo take place. It is a singularity of 
sponges, without a parallel among other animals, that the egg 
becomes the embryo without quitting its seat of formation—the 
follicle in which it grows up. 
The sponge larva escapes from the body of the parent by 
TE, Ray Lankester, Quart. Journ. Micros. Sci. 1879. 
