SPONGES 69 



very large granular cells with vesicular nuclei, which represent 

 undifferentiated blastomeres and are destined to give rise to the 

 archaeocytes, and therefore also to the sexual cells of the adult. 

 The flagellated cells, on the other hand, are the ancestors of all 

 the tissue-forming cells of the adult. 



The larva is hatched either in this condition or by retardation 

 at a stage slightly in advance of it and swims freely for about 

 twenty-four hours, first at the surface of the water and then near 

 the bottom. Meanwhile, a new class of cell-elements is being formed 

 by modification and immigration of individual cells of the flagellated 

 parietal layer (Figs. 57 and 58, 1). Here and there a flagellated cell 

 is observed to retract its flagellum, while its nucleus undergoes an 

 alteration in shape and structure, becoming spherical, with more 

 evenly distributed chromatin and with a nucleolus. The cell at 

 the same time becomes more compact, draws in its more external 

 portion, and finally migrates from the body wall into the internal 

 cavity of the larva (Fig. 58, l''-!''). As the result of this process, 

 repeated often and at all points in the ciliated layer, with the 

 exception of the extreme anterior pole, the larval cavity becomes 

 fiUecl with a mass of amoeboid cells, and the larva itself shrinks 

 considerably in size. By the second day the larva, which is now 

 ripe for fixation, has become a compact, planula-like organism, con- 

 sisting of three kinds of cells : (1) The external layer of flagellated 

 cells, destined to become the gastral layer ; (2) an inner mass of 

 amoeboid cells, the future dermal layer ; and (3) the two .still un- 

 changed posterior granular cells. Larvae of this type are termed 

 parenckymulae, and are found in the family Clathrinidae and in some 

 Heterocoela. 



The larva fixes by the anterior pole, or by one side, and under- 

 goes a complete change of form and appearance, becoming a flattened 

 plate with irregular amoeboid contours (Fig. 57, 2-'). In fact, at the 

 metamorphosis it resembles nothing so much as a small Amoeba, 

 whereas when free swimming it might have been mistaken for an 

 Infusorian. At first cell-outlines are not clearly distinguishable on 

 the surface, but towards the end of the first day of fixation the 

 surface can be seen to be covered by a distinct layer of flat 

 epithelium (Fig. 57, 2^). The metamorphosis of the larva, when 

 complete, is eft'ected by means of radical changes in the relative 

 positions and functions of the different cell-elements of the body 

 (Fig. 58, 2). The majority of the cells of the inner mass of the larva 

 have passed out to the exterior and acquired a superficial position, 

 forming an epithelial layer, the future dermal epithelium enclosing the 

 fcjrmerly external ciliated layer. This reversal of position is effected 

 partly by dehiscence, the inner mass bursting out at some part of the 

 larva and growing round the disrupted ciliated layer, and partly by 

 diapedesis, the individual amoeboid cells struggling through the 



