SPONGES 71 



ciliated layer to the exterior. The epithelium of the upper surface 

 and edges is formed by the first method (overgrowth), that of the 

 central portion of the under surface chiefly by the second method 

 (undergrowth). The ciliated cells of the larva have lost their 

 characteristic form, becoming simply rounded, Avith an irregularly 

 shaped nucleus attached to one side of the cell ; they lie huddled 

 together in a compact mass in the interior, and hence their flagella 

 are very difficult to make out. Scattered amongst the ciliated cells 

 are a certain number of cells of the larval inner mass which still 

 remain in the interior and are destined to become the future 

 porocytes. The greatest change is that undergone by the two 

 posterior granular cells, Avhich have become broken up into a great 

 number of small corpuscles of peculiar aspect rather resembling some 

 varieties of leucocj'tes. As a result of all these changes the pupa 

 at the completion of metamorphosis, i.e. towards the end of the 

 first day of fixation, consists of the following cell-elements : (1) An 

 external, flat epithelium, derived from the inner mass of the larva, 

 enclosing (2) a compact mass of cells, the formerly external ciliated 

 cells of the larva, amongst which are (3) a few porocytes, derived 

 from the larval inner mass, and (4) a great number of minute amoe- 

 bocytes, derived from the two jDOSterior granular cells of the larva. 



The subsequent development is comparatively simple. On the 

 second day of fixation the pupa becomes more compact, and by 

 drawing in its marginal pseudopodia, assumes the form of a bun or 

 cake (Fig. 58, 3). At the same time, a number of the superficial 

 dermal cells have migrated inwards from the epithelium and taken 

 up a position immediately beneath it, where they become grouped 

 in trios to form the triradiate spicules, which arise exactly as in the 

 adult (Fig. 58, 3, skel). In this way is initiated the division of the 

 dermal layer into the external contractile and the internal skeleto- 

 genous strata. The porocytes meanwhile have become grouped 

 together in the interior of the pupa. The results of these changes 

 are better seen on the third day (Fig. 57, 3), when the young spicules 

 beneath the epithelium have become very obvious ; and at the same 

 time the future gastral cavity has made its appearance as a more 

 or less irregular space, or spaces, in the middle of the centrally 

 placed porocytes, which at first form a continuous epithelium lining 

 the cavity (Fig. 58, 4). 



Towards the end of the third day the further enlargement of 

 the gastral cavity causes the cells of the porocytic epithelium lining it 

 to become separated and isolated from one another, so that the 

 gastral cells come to form the boundary of the cavity. On the 

 fourth day the pupa has grown in height, chiefly by the develop- 

 ment of a now spacious gastral cavity, round which the gastral 

 cells form in most places a single layer (Fig. 58, 5). The porocytes 

 are migrating outwards, and are found either between the gastral 



