DEVELOPMENT 



173 







1. 



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

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

 on its axis. 



Sections show that the larva is built up of two layers : 



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

 elatinous 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, which possesses a nucleolus ; 

 these are the archaeo- 

 cytes; (2) differentiated 

 cells, of which the nucleus 

 contains a chromatin net ; 

 these give rise to pinaco- 

 cytes, collencytes, and 

 seleroblasts. Some of them 

 form a flat epithelium, 

 which covers the hinder 

 pole. Some of the sclero- 

 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 the most important process of the meta- 

 morphosis ; it is followed by a pause in the outward changes, 

 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 ; the flagellated cells, which have lain in con- 

 fusion, become grouped in small clusters. These become flagellated 

 chambers, communications are established between the various 

 portions of the canal system, and its external apertures arise. 

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

 by 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 keep the temperature constant. They will undergo meta- 



Fig. 68. Longitudinal section through the hinder 

 pole of the larva of G. varius. a, Flagellated 

 cells ; ma\ undifferentiated cell ; ma 2 , differen- 

 tiated cell ; pi, pigment ; x, surface of hinder 

 pole. (After Maas.) 



