106 LIFE HISTORIES OF THE SPONGES. 



into their interior, not only fluid, but also minutely diffused solid 

 matter (e. g., carmine), probably through the soft exoplasm be- 

 tween the collar and flagellum. Liberated artificially, they also 

 assume amoeboid shapes and motions. On the endodermal cells 

 devolve the whole of the nutritive (digestive, respiratory and 

 secretory) functions ; and there can be little doubt that both eggs 

 and spermatozoa are modified endodermal cells." 



Hseckel did not observe the development of the larva, his gas- 

 trula, into the young sponge. This gap has been filled by Metsch- 

 nikoff. He observed the course of development in Sycon ciliatum 

 (Fig. 50) from the segmentation of the yolk, through the larval 

 state, up to the time when the sponge is fixed and the spicules are 



well developed ; in fact, through nearly every important stage in its 

 life. By making a section through the sponge he found eggs and 

 embryos in different stages of development in springtime. The 

 total segmentation occurred as Haeckel describes. Metschnikoff, 

 however, observed that a small '» segmentation-cavity " appeared 

 in the egg (Fig. 50, A, c) which soon disappeared (Fig. 50, B). 

 As a result of the process of division, a roundish embryo appears, 

 which is made up of a large number of small cells. He was un- 

 able to study the mode of origin of the germ-layers. The free- 

 swimming larva (Fig. 50, C) is an oval body, made up of two 

 sorts of cells : those which are small, long and ciliated, and cer- 

 tain large round ones, much fewer in number. The first form a 



