304 H'ILSOjV. [Vol. IX. 



The two mesodermic processes, in. p., do not come to an 

 abrupt end, but die out imperceptibly. The ectodermal mem- 

 brane, from the very beginning of its appearance, becomes 

 covered with debris, much of which is organic. In Fig. 54, 

 round the main body of the sponge are strewn little masses of 

 degenerating cells, dead protozoa, and homogeneous, rounded 

 masses which betray their organic nature only by their affinity 

 for the stain, and which are evidently degeneration products. 

 A good part of this debris looks as if it came from the sponge 

 itself, as if it were composed of cells which had lost their 

 connection with the sponge in some way, and then degenerated. 

 Before completing the description of the ectodermal membrane, 

 PI. XVIII, Fig. 57, needs a word of explanation. The sponge 

 was surrounded by an ectodermal membrane of average width, 

 which in the region drawn was thrown, perhaps artificially, 

 into a fold, x, close to the edge of the mes-entoderm. 



My description of the peripheral region of the young sponge 

 differs from that given by Maas for Spongilla (14), which he 

 finds can also be applied to the case of Esperia (16). In the 

 accuracy of my own observations as far as they go, I am 

 confident ; and those of Maas seem to have been made with 

 such care that I am inclined to believe farther study will 

 reconcile the two descriptions. 



In the young Spongilla, Maas observed that the whole 

 periphery becomes amoeboid. The formation of processes was 

 followed in the living sponge, and it could be seen that a 

 hyaline prolongation was thrown out far beyond the inner 

 tissues, into which the inner cells slowly flowed, the outline 

 then becoming more even. To this outer region, which is in 

 constant motion, Maas gives the name of "der amoeboide 

 Hof." The amoeboid "Hof" forms the peripheral zone of 

 the sponge, and is composed of ectoderm. Internal to it is 

 the body of the sponge, /.f.,the mass of mes-entoderm. The 

 movements of the amoeboid " Hof " are due to the amoeboid 

 movements of its constituent (ectoderm) cells. Silver nitrate 

 preparations show that the cells at the extreme edge of the 

 Hof are in active motion, throwing out pseudopodia, and com- 

 bining to form lobes (the hyaline prolongations, the formation 



