No. 3.] DEVELOPMENT OF MARINE SPONGES. 329 
sponding structure in Esperella, by the compression of the 
surrounding tissue owing to the growth of the gemmule. 
Development of Gemmule into Swimming Larva.— The 
mature gemmule breaks up into masses, and these into smaller 
masses, and so on until the entire gemmule has been resolved 
into distinct cells, as in the development of Esperella. The 
process is odder and more striking looking in Tedania, owing 
to the absence of any indication of the individual cells and 
owing to the extreme irregularity of the first fissures, Fig. 72 
(section through a gemmule just beginning to break up). In 
Fig. 73 is shown a portion of a section through another gem- 
mule which has already broken up into small masses of varying 
size, even in the smallest of which nuclei are as yet invisible. 
In a stage a little later, Fig. 74 (the entire embryo was spher- 
oidal), nuclei make their first appearance. In this embryo the 
division of the gemmule masses has been carried so far that 
the individual cells are easily recognisable. The superficial 
cells are packed tightly enough to make a continuous layer, 
which will become the ectoderm, inside which are scattered 
cells and rounded masses, separated by a clear fluid and 
more or less united by delicate protoplasmic processes. The 
bodies of the cells and rounded masses are just as full of the 
finely granular yolk as was the mature gemmule, and nuclei 
are only visible in some of the cells and a few of the masses. 
The masses are usually divided into rounded lobes and are 
obviously about to split up into individual cells. 
The hitherto spherical embryo begins to assume an oval 
shape. In Pl. XXI, Fig. 75, is shown part of a section through 
a roughly oval embryo, in which the differentiation of the 
layers is noticeably more advanced than in Pl. XX, Fig. 74. 
Nuclei are apparent in all the cells, and the ectoderm is more 
distinctly marked off from the inner mass of cells (mes- 
entoderm). In some of the ectoderm cells two nuclei can be 
seen, indicating that cell division is taking place. The mes- 
entoderm in this stage consists of a very loose network of cells 
connected together by long slender protoplasmic processes. 
Many of the mes-entoderm cells are closely packed in dense 
groups, and there are a number of multilobed and often multi- 
