FIELD BOOK OF PONDS AND STREAMS 



should be added so gently that the gemmules will not be 

 shaken. Within a day or two they stick to the glass, no longer 

 rolling with the slightest tremor of the water. This is the 

 first sign of growth, and shows that the soft sponge cells have 



Fig. 85. — Sponge colon}'- 3 days old with the gem- 

 mules from which it has grown. Note chimney- 

 like elevations. 



grown out through the little hole in the gemmule shell, over 

 its side and onto the glass. They make the white plug which 

 shows in the hole and the white drift around the dark gem- 

 mule (Fig. 85). By the time the colony has been growing 

 three or four days little elevations will appear on its surface. 

 At first their tips are translucent bulbs but these soon break 

 through and the whole structure becomes chimney-like with 



Fig. 86. — Beginnings of the sponge skeleton; the 

 spicules of silica (opal) showing on the edge of a 

 colony 6 days old. 



no 



