No. 3-] DEVELOPMEXT OF MARIXE SPOXGES. 315 



shown lying near one another in the pillars of tissue separating 

 the subdermal cavities. Other long straight spicules are scat- 

 tered freely about in the deeper parts of the body. The bow- 

 shaped spicules present in the swimming larva are found in 

 small numbers distributed irregularly through the mesoderm 

 of the attached sponge, Fig. 44. The embryonic shovels which 

 in the swimming larva are united in rosettes, PI. XVI, Fig. 30, 

 and PI. XVn, Fig. 34, are always found free in the attached 

 larva. In the young sponges there are not many of these 

 spicules, and the few to be seen are usually found in the 

 dermal membrane, PI. XVII, Figs. 44, 50. 



Sunnimiy of the Leading Facts in the Gcvtviulc Development 

 of Esperclla. 



1. Gemmules appear in any part of the sponge mesoderm, 

 and when present in large numbers, cause degeneration in the 

 sponge tissue. 



2. A number of mesoderm cells well supplied with yolk 

 collect together and the mass so formed rounds itself off into a 

 gemmule, the outer cells becoming the follicle. 



3. The gemmule grows not only by cell division, but by the 

 fusion with it of other small gemmules. It becomes a large 

 mass of closely packed cells, full of fine yolk. 



4. The gemmule, when mature, breaks up into irregular 

 masses of cells, and these separate into the constituent individ- 

 ual cells. 



5. The outer cells become ectoderm. Those at the posterior 

 pole flatten, and develop neither flagella nor pigment. The 

 other ectoderm cells become columnar, and develop both 

 flagella and pigment. 



6. The inner mass of cells forms an intercellular network. 

 It is a parenchyma in which there is no distinction between an 

 ectoderm and a mesoderm. The parenchyma cells at the 

 posterior pole become closely compressed. 



7. In the swimming larva there is a bundle of long straight 

 spicules in the posterior end. Bow-shaped spicules and embry- 

 onic shovels (in rosettes) are scattered through the parenchyma. 



