STRUCTURE OF THE LARVA OP SPONGILLA LACUSTRIS. 459 



there is a group of granular cells placed at the posterior pole 

 in the pseudogastrula stage, and afterwards lodged in the 

 interior of the larva. The granular cells in question are none 

 other than the mother cells of the amoeboid wandering cells, 

 i. e. of the cells which later on become, either wholly or in part, 

 the reproductive cells. Clearly, therefore, the first division of 

 labour occurring among the cells of the Ascon larva is the 

 differentiation of locomotor and reproductive cells respectively. 

 Further, Mr. Minchin believes, and I fully agree with him, 

 that the same differentiation takes place in the Sycons. The 

 few granular cells at the posterior end in the youngest Sycon 

 embryo (pseudogastrula) are the same as those found in the 

 Ascon larva3, and they are situated, at the time of the invagi- 

 nation of the blastula to form the so-called pseudogastrula, 

 adjacent to the almost obliterated cavity of the blastula, — that 

 is, they are the innermost cells of all. The other non-ciliated 

 cells are developed later from the ciliated layer at the posterior 

 pole, thus producing the characteristic comphiblastula larva, 

 composed of ciliated cells anteriorly, non-ciliated (dermal) cells 

 posteriorly, and a mass of granular cells in the centre. The 

 central cells of the larva probably become the amoeboid cells of 

 the adult sponge, i. e. they become the cells which will give rise 

 to the reproductive cells. If this interpretation be true, the first 

 division of labour in the Sycons would be into mother cells of 

 the sexual cells on the one hand, and ciliated cells on the other, 

 the cells which are destined to become the dermal layer 

 developing later from the ciliated cells, in the same way as 

 has been described in Leucosolenia variabilis by Mr. 

 Minchin (8). It is highly probable that the methods of histo- 

 genesis which take place in the Ascon and Sycon larvae are 

 essentially similar. The point I wish to emphasise, however, is 

 that the first cell-differentiation in these larvae, is into ciliated 

 cells on the one hand, and reproductive cells on the other, the 

 latter being represented by the posterior granular cells. 

 Among the Metazoa this does not always occur; the repro- 

 ductive cells or their immediate precursors appear early in 

 the development only in a few cases. As a rule, epiblast and 



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