3i6 McMURRICH. [Vol. IV. 



to be here a difficulty, in the fact that immigration is not accom- 

 panied by a division of the cell. Immigration, however, must 

 necessitate a considerable expenditure of energy, by the cell in 

 general, which energy, it seems quite conceivable, might be 

 turned, when immigration became unnecessary or impossible, 

 to the production of a division. 



Supposing the delamination to have arisen in this way in the 

 lower Actinozoa, it is probable that on a subsequent loss of 

 food-yolk by the more recent forms, delamination would persist 

 as the mode of formation of the central mass or endoderm, even 

 though a large blastocoel was present in the blastula. 



I believe then that the phenomena occurring in Metridium 

 are to be explained on the supposition that the Hexactiniae are 

 descended from forms whose ova possessed a relatively large 

 amount of food-yolk. What we find in the Alcyonaria is the 

 original condition, and the more typical delamination of the 

 Hexactiniae has been derived from this. In Renilla the central 

 cells, heavily laden with food-yolk, are separated from the pro- 

 portionately more protoplasmic ectoderm ; later, a certain num- 

 ber of the central cells become transformed into the endoderm 

 layer, while the rest degenerate and their contents serve as food 

 for the developing embryo, being absorbed by its endoderm 

 cells. So too, though to a less extent, in Manicijia ; the yolk is 

 here much less in quantity, but still the same processes occur as 

 in Renilla. In Metridium the yolk is much more reduced, so 

 that the blastula cavity never becomes filled up, the delami- 

 nated cells becoming almost entirely converted into the defini- 

 tive endoderm, a few only disintegrating, and giving rise to the 

 granules of food-yolk which lie in the endodermal cavity. 



2. The Formation of the First Eight Mesenteries in 



Rhodactis. 



The embryos of Rhodactis Sancti-ThomcE, when extruded by 

 the parent, have, as stated above, from two to four perfect mes- 

 enteries. In shape they are pyriform, the mouth being situated 

 at the narrower pole. The ectoderm is high, and shows a differ- 

 entiation into the various kinds of cells usually to be found in 

 that layer in the Actinians, but does not yet present that pecul- 

 iar vacuolated appearance, due to an excessive development 



