DEVELOPMENT 1 73 



meiit takes place in a screw line ; when this ceases the larva 

 rests on its hinder pole, and tlie cilia cause it to turn round 

 on its axis. 



Sections sliow that tlie larva is built up of two layers : — 



1. " The inner mass," consisting of various kiiuls of cells in a 

 gelatinous matrix. 



2. A high flagellated epithelium, which entirely covers the 

 larva with the exception of the hinder pole. 



The cells in the inner mass are classified into (1) undifferenti- 

 ated cells, recognised by their nucleus, wliich possesses a nucleolus ; 

 these are the archaeo- 

 cytes; (2) differentiated 

 cells, of which tlie nucleus 

 contains a chromatin net : 

 these give rise to pinaco- 

 cytes, collencytes, and 

 scleroblasts. Some of them -^ 



form a flat epithelium ^"^' ^^' — Longitudinal section through the hinder 



1 1 • 1 ' P°^^ °^ ^^^^ ^''"'^''''' °f ^- varius. a, Flagellated 



which covers the hinder cells ; ma^, undifferentiated cell ; mu\ differen- 



P-.le. Some of the sclero- t''^**^*^ '/"j ^'(v i''^"'"'"* ' •"' '"^''^^'^ °^ ^""'''^'" 



^ _ pole. (Alter Maas.) 



blasts already contain 



spicules. Fixation occurs very early. The front pole is used 

 for attachment, the pigmented pole becoming the distal end 

 (Fig. 69). The larva flattens out, the margin of the attached 

 end is produced into radiating pseudopodial processes. The 

 flagellated cells retreat to the interior, leaving the inner mass 

 exposed, and some of its cells thereupon form a flat outer 

 epithelium. This is tlie most important process of the meta- 

 morphosis ; it is followed by a pause in the outward ch.uiges, 

 coinciding in time with rearrangements of the internal cells to 

 give rise to the canal system ; that is to say, lacunae arise in the 

 inner mass, pinacocytes pass to the surface of the lacunae, and 

 form their lining ; tlie flagellated cells, which have lain in con- 

 fusion, become grouped in small clusters. These become flagellated 

 chambers, communications are established between the various 

 ])ortions of the canal system, and its external apertures arise. 

 There is at first only one osculum. The larvae may be obtained 

 l»y keeping the parent sponge in a dish of sea. water, shielded 

 from too bright a light, and surrounded by a second dish of 

 water to kecj) the temperatun^ constant. Tbey will undergo meta- 



