SPONGES 73 



(1) Cell fnultiplication or segmentation of the ovum. 



(2) Primary cell differentiation into tissue-forming cells (histo- 

 cytes) and primordial or reproductive cells (archaeocytes). 



(3) Secondary cell differentiation or separation of the histocytes 

 into two primary germ layers (blastogenesis). 



. (4) Rearrangement of the cell layers in accordance with their 

 disposition in the adult (metamorphosis). 



(5) Tertiary cell differentiation or tissue formation (histogenesis). 



(6) Growth and acquisition of the body form (morphogenesis). 

 In Clathrina these six processes follow one another in the order 



here indicated, the first and second taking place during the embry- 

 onic period, the third during the larval period, the fourth at fixa- 

 tion, and the fifth and sixth, more or less intermingled, during the 

 pupal period. We shall find that the great apparent differences be- 

 tween the various types of sponge development are in the main the 

 outcome of changes in the order in which these processes occur, and 

 in their relation to the three periods of development, such changes 

 being combined with specific or morphological characters of compara- 

 tively slight importance. For instance, all cell differentiation may 

 be thrown back to the embryonic period, thus coming to precede 

 the metamorphosis, and in such cases the larval period is rendered 

 barren, so far as developmental processes are concerned, and may 

 be greatly shortened, lasting only a few hours. In some Ascons, 

 on the other hand, e.g. Clathrina cerebrum, the pelagic larva may 

 swim at the surface for three or four days. 



(/3) Types of Sponge Larvae. In the absence of any knowledge of 

 the developmental history of the Hexactinellids, we may consider 

 first the Calcarea and then the Demospongiae. A very instructive 

 evolutionary series is furnished by the larvae of calcareous 

 sponges, for which the larva of Clathrina blanca, described above, 

 may serve as a convenient starting-point. 



The larvae of other Clathrinidae are parenchymulae very similar to that 

 of Cl blanca, but exhibiting variations in two important features. In the 

 first place, the conspicuous posterior granular cells may vary in number 

 in different species, there being perhaps only one, or as many as four, 

 or even a yet larger number in some cases ; or, on the other hand, they 

 may be absent altogether, the body wall being made up entirely of 

 ciliated cells. The latter condition is due in reality to the cells in 

 question having become broken up into minute amoebocytes before the 

 larval period instead of after fixation, and in such cases the inner mass 

 of the larva contains two kinds of cells, which were regarded by Metsch- 

 nikoff as "endoderm" and lt mesoderm " respectively. It is interest- 

 ing to note that all these variations in the condition of the posterior 

 granular cells or amoebocytes may occur as abnormalities in one species 

 (e.g. Clathrina blanca}. 



In the second place, the apparent absence of posterior granular cells in 



