3o6 IV/LSOX. [Vol. IX. 



cells too take part in this intercellular network. In PI. XVII, 

 Figs. 39 and 42, are shown parts of vertical sections through 

 two sponges in the same stage as Fig. 38. Owing to the 

 quantity of water in the sponge at this age, the tissues are 

 extremely delicate and gelatinous, and the intercellular net- 

 work in the best preparations is naturally more or less broken, 

 with many of the cells fallen out of their proper places. That 

 the canals and subdermal cavities arise as great intercellular 

 spaces or lacunae in the mes-entoderm, can easily be seen in 

 these figures. The lacunae when first established have no 

 definite walls, but are merely surrounded by ordinary undiffer- 

 entiated mes-entoderm cells, cati., Fig. 39. The cells imme- 

 diately surrounding the cavity then begin to flatten, throwing out 

 lateral processes in such a way as to form a more or less com- 

 plete wall, in which, however, the component cells are of very 

 irregular and diverse shapes, Fig. 42, and can., PL XVIII, Fig. 

 47 (small part of a section, such as PI. XVII, F"ig. 44). The 

 lining cells continue to flatten, ultimately forming a continuous 

 investment of epithelioid cells, so thin indeed that they consti- 

 tute nothing more than a nucleated membrane, can. zc, Fig. 47. 

 In Fig. 44 there is shown a small canal, can.', in which a part 

 of the wall has reached the condition of a nucleated membrane, 

 while the other part is still composed of cells which have not 

 yet flattened out to any great degree. Cavities are developed 

 everywhere directly beneath the upper surface, and there con- 

 stitute, as has been said, the system of subdermal cavities, PL 

 XVII, Figs. 38, 44, and 50; PL XVIII, Figs. 48, 51, 52, and 53. 

 The spaces formed deeper in the tissue of the sponge become 

 the canals. The number of subdermal cavities and canals is at 

 first relatively small, so that the space occupied by the meso- 

 derm is comparatively great, PL XVII, Figs. 38 and 44. But 

 as new canals are formed, and as the cavities and canals 

 gradually connect with one another, the mesoderm becomes 

 reduced in quantity, and before long assumes the adult con- 

 dition, in which it consists of uniformly thin trabeculae 

 separating the various canals. The increase in the extent of the 

 series of cavities may be seen in a comparison of PL XVII, Fig. 

 44, with sections through older sponges, Pis. XVII and XVIII, 



