84 SPONGES 



elements arise from the primitive cells of the inner mass is exactly com- 

 parable to the origin of all the different kinds of tissue from the one kind 

 of cell-element contained in the gemmule, or to the differentiation of a 

 larva from the mass of uniform blastomeres derived from segmentation of 

 the ovum ; and it is probable that this aberrant feature in the larval 

 development of Spongillinae is correlated with the acquisition by these 

 sponges' of the method of reproduction by means of gemmules, the 

 peculiarities of which have been, or are being, acquired by the larvae also 

 to a greater or less extent. 



The main features of sponge embryology may be summarised as 

 follows : 



I. The larva is composed of three classes of cell-elements: (1) 

 Columnar flagellated cells, forming the outer covering or localised 

 at the anterior pole ; (2) rounded, more or less amoeboid elements, 

 rarely flagellated, forming the inner mass or aggregated at the 

 posterior pole ; and (3) the archaeocytes, usually scattered in the 

 inner mass and often represented by undifferentiated blastomeres. 



(a) In the more primitive types the primary differentiation of 

 the cells is into (1) flagellated cells (histocytes), and (3) primor- 

 dial cells (archaeocytes), and the cells of the inner mass (2) arise 

 by modification of a certain number of flagellated cells, others re- 

 maining unmodified as the flagellated cells of the ripe larva. 



(b) In less primitive types the blastomeres of the ovum become 

 differentiated in situ into flagellated cells, archaeocytes, and cells of 

 the inner mass, the last named becoming still further differentiated 

 histogenetically before or during the larval period. 



II. The larva fixes and undergoes a metamorphosis whereby 

 the flagellated cells become placed in the interior, while the cells of 

 the inner mass come to surround them completely. 



III. (1) The flagellated cells of the larva become the collar cells of 

 the adult (gastral layer), acquiring a collar. No other tissue elements 

 arise from them, but some (or all T) of the ciliated chambers may 

 arise secondarily from undifferentiated blastomeres or archaeocytes 

 (Spongilla) ; (2) the inner mass gives rise to the dermal layer in its 

 entirety, that is to say, to the whole of the flat epithelium, the poro- 

 cytes, and the connective tissue layer of the adult ; (3) the archaeo- 

 cytes become the wandering cells of the adult, from which the 

 reproductive cells arise. 



With regard to the transformation of larval flagellated cells into 

 the collar cells of the adult, it should be borne in mind that the 

 collar is specially developed when the sponge is actively feeding 

 and becomes completely retracted when at rest. Hence its absence 

 in the larva may be explained by the fact that the nutritive 

 functions are temporarily in abeyance. Taking this fact into 

 account, it is evident that the characteristic collar cells of sponges 

 are direct derivatives, only modified in unimportant details of shape, 



