94 RICHARD EVANS. 



side of the central cells^ i. e. on the side opposite the point 

 which later on will be occupied by the pore, and all three 

 increase in extent in a similar way. They ultimately form 

 complete layers, though one of them, viz. the columnar layer, 

 is no longer found in the mature gemmule. 



The migration of scleroblasts, or cells that would become 

 scleroblasts, is not a new idea to zoological literature. Mr. 

 Bourne described such migration of the calicoblasts in Helio- 

 pora coerulea (1), and Professor Minchin has given a full 

 account of the migration of the epithelial cells in the Ascons 

 to the interior, and the subsequent formation of spicules inside 

 them (16). It is true that in both these cases the migration 

 to the interior is previous to the formation of spicules, while 

 in Ephydatia blembingia the amphidiscs are fully formed 

 before the change of position takes place. This difference 

 does not in any way tend to minimise the importance of the 

 facts described above. The amphidiscs are so small as com- 

 pared with ordinary spicules, and their ends are rounded, con- 

 sequently there is no inherent improbability in the view that 

 they are carried from one place to another by the scleroblasts. 



(4) The Origin, Structure, and History of the 

 Trophocytes. — The trophocytes are large round cells with 

 vesicular nuclei, the chromatin of which is for the most part 

 aggregated in small granules either round the spherical 

 central corpuscle or against the nuclear membrane, the inter- 

 vening space being, as a rule, occupied by only a few small 

 granules. In the immediate neighbourhood of the nucleus 

 there are innumerable small and irregularly shaped granules 

 which give the cell a dirty-looking appearance, the peripheral 

 portion being exceptionally clear and devoid of granules of 

 any kind. A negative feature of these cells is seen in the 

 absence of both yolk bodies and nutritive vacuoles. 



The trophocytes originate from the sponge as a separate 

 class of cells, like the three other classes which have been 

 already considered. They migrate from the sponge tissue at 

 the same time as, and along with, the cells which become 

 columnar. While the columnar cells always remain outside 



