142 Mr. W. S. Kent oyi the Emhryology of Sjponges. 



centre of the entire body, which is thus seen as though in 

 longitudinal section (PL VI. fig. 10) , it will be found that the 

 constituent cellular units or segment-masses have assumed an 

 elongate conical contour, gradually tapering from the exposed 

 peripheral border : the same being united by their posterior ex- 

 tremities, and closely adpressed to one another throughout 

 their lateral extent, they, as it were, in fact, radiate from a 

 common centre. Under these same conditions it is clearly 

 shown that a single cilium originates from the centre of the 

 peripheral border of each of these elongate units, and from its 

 great proportional length may be more correctly designated 

 a flagellum. Increasing in size, it is next found that these 

 elongate units become separated posteriorly, leaving a central 

 ovate or spherical cavity in the common body, while at the 

 same time a short hyaline cup-like expansion develops around 

 the base of the flagellum. This stage is represented in fig. 11 

 of the same Plate, and is also admitted in Barrois's drawings, 

 and, with some slight modification, in those of Haeckel also. 

 Upon this last there now, however, succeeds a phase wliich 

 so far has apparently been overlooked by other observers, 

 though it has been encountered personally in association 

 with numerous sponge-forms, and constitutes, in fact, in 

 accordance with the views here adopted as to the nature 

 of these organisms, a natural sequence to the preceding. 

 The aspect now presented is delineated at PI. VI. fig. 12 — the 

 gemmule at this point of the development, as will be at once 

 recognized, consisting of an ovoid aggregation of closely 

 joined collar-bearing units in no way differing individually 

 from the typical collar-bearing sponge-monads or spongozoa 

 of which the adult sponge-body is composed. Each separate 

 unit of this ovoid mass is at this stage of its existence a per- 

 fect individual collar-bearing monad, taking in an independent 

 food-supply, which it captures with its collar of adhesive circu- 

 lating sarcode in a manner similar to that already described by 

 me of Monosiga gracilis and other free collar-bearing monads, 

 in the last January number of the ' Annals ' *. The morpho- 

 logical identity of the individual units of the sponge-embryo 

 or gemmule with those of the independent monads alluded to 

 becomes at once patent on placing side by side, as I have done 

 at PI. VI. figs. 13 and 14, the simple flagellate and adult 

 collar-bearing condition of an independent freshwater monad, 

 * Mr. Xenos Clark, of the San-Francisco Microscopical Society, from 

 wliom I have just received a very complimentary acknowledgment of 

 my recognition and support of his father's, the late Prof. H. James- 

 Clark's, discoveries and theory concerning the nature of sponges, has very 

 appropriately compared this sarcode-circulation of the hyaline collar as 

 discovered hv me to the action of an ''endless revolving belt." 



