44 ZOOLOGY. 
the formation of a dlastula, and then a three-layered sac, cor- 
responding to the gastrula of the higher animals. In this 
state (Fig. 30a) the germ breaks out of the parent sponge into 
the sea. Fig. 31 represents the development of the common 
little calcareous sporge (Sycon ciliatum), found between 
tide-marks. A indicates the morula with the segmentation- 
cavity (c), which afterward 
disappears as at B. The 
blastula is represented at. 
C, and consists of ciliat- 
ed and non-ciliated large 
round cells ; the first series 
Fig. 30.—-Segmentation of egg of sponge forming a sort of arch, with 
sg seas aa a hollow in the middle, 
around which a large number of very fine brown pigment 
corpuscles are collected. The next change of importance is 
the disappearance of the cavity, the upper or ciliated half 
of the body being much reduced in size. Then the large 
round cells of the hinder part are united into a compact 
mass, leaving only a single row. The ciliated cells are 
gradually withdrawn into the 
body-cavity. Fig. 31, D, shows 
the gastrula condition. At this 
period also the larva becomes ses- 
sile, and now begins the formation 
of the sponge-spicules, which de- 
velop from the non-ciliated round 
cells. Metschnikoff calls atten- 
tion to the fact that at this early 
stage the Sycon passes through a 
phase which is persistent in the p., sq, 
i —Blastula ofa sponge (Sy- 
genus Sycyssa. The layer of cil- candra raphanus).—After Schulze, 
iated cells are gradually withdrawn into the body-cavity, 
until a small opening is left surrounded with a circle of 
cilia. These cilia finally disappear, a few more spicules 
grow out, and meanwhile the opening disappears. In the 
gastrula (represented at D) a considerable body-cavity ap- 
convenience to avoid circumlocution. It may be that these conditions 
will be found to be essentially modified in different groups of animals. 
