No. 3-] DEVELOPMENT OF MARINE SPONGES. 329 



spending structure in Esperella, by the compression of the 

 surrounding tissue owing to the growth of the gemmule. 



Development of Gemtiiule into Siviniming Larua. — 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 

 F'g- 73 's 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 PI. 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- 



