No. 3-] DEVELOPMENT OF MARINE SPONGES. 303 



56, 57). After the sponge takes on the round cake-like shape, 

 the mes-entoderm becomes divided into two regions (Fig. 55), 

 a main central region in which the formative cells are more or 

 less rounded and pretty densely packed, and a peripheral zone 

 in which the formative cells become branched and amoeboid, 

 and in which they are loosely packed. The distinction between 

 the two regions is conspicuous in Fig. 55, drawn under a low 

 power ; and the manner in which the peripheral zone is formed 

 is clearly seen in Fig. 56 (surface view of a part of the 

 peripheral region of a young sponge). Further development 

 bestows on the peripheral zone the character of an exquisite 

 intercellular network (see/, z.. Fig. 57, surface view of a small 

 part of the peripheral region of a young sponge). In the 

 sponge represented in Fig. 57, the cells of the network are of 

 about the same size as the formative cells in the rest of the 

 body; but as the sponge grows older, the cells of the peripheral 

 zone grow smaller, many of them becoming delicate spindle- 

 shaped cells. The peripheral zone after it has assumed this 

 character, is shown more or less well in all the sections of older 

 sponges figured (see PL XVII, Fig. 44, and especially PL XVHI, 

 Fig. 49, the peripheral part of a section such as Fig. 44). The 

 processes of many of the cells run directly into the ectodermal 

 membrane, and strongly suggest an intercellular connection 

 between the ectoderm and mesoderm in this region. 



When the ectodermal membrane grows out round the periph- 

 ery of the sponge, the peripheral zone of mes-entodermic 

 cells, which is already differentiated from the central mass, 

 begins to push out lobes and processes within the membrane 

 which is at its inner edge obviously composed of two layers 

 (Fig. 49). This brings it about that the inner mass of cells 

 loses the smooth contour of younger stages (Fig. 55), its 

 edge becoming, instead, jagged and irregular (Fig. 58). The 

 changes of shape which the sponge undergoes at this time are 

 due to the fluctuations of the edge of the mes-entoderm mass, 

 not to amoeboid movements of the ectoderm cells. Sponges 

 are sometimes found in which the mes-entoderm has pushed 

 out processes of very considerable length between the layers of 

 the membrane. Such a sponge is shown in PL XVTII, Fig. 54. 



