54 The Unity of the Organism 



That such a mode of development is entirely foreign to crystal 

 production hardly needs to be remarked. But if further proof 

 to the same effect were demanded^ one other strong piece of evi- 

 dence is the fact that most of the spicules are not composed of 

 inorganic substance alone but have a core of organic matter. 



Although the technique for the examination of cytoplasm de- 

 veloped during these last years has not, so far as I know, been 

 applied to the spicule-producing cells of sponges, we can be 

 reasonably sure from the study of other secretory cells what the 

 general results will be when such application sliall have been 

 made. They will bring out numerous details not now known of 

 how both the nucleus and the cytoplasm act during spicule pro- 

 duction. 



Surely it is not necessary for me to dwell again on the 

 main point of the eyidence here presented. The nuclei of 

 the spicule-fomiing cells may take an active part in pro- 

 ducing the spicules. Indeed from our general knowledge of 

 nuclear activities, illustrations of which were given in an 

 earlier chapter, it is probable that such will some day be 

 demonstrated to be the case. But the proof of nuclear 

 activity in spicule production will not he disproof of the 

 already observed cytoplasmic activity in spicule production. 



Other animals that may be mentioned in which spicules 

 are produced in much the same way and have the same taxo- 

 nomic diversity and constancy are the alcyonaria among 

 coelenterates, the holothurians among ecliinoderms, and 

 some of the compound ascidians, particularly of the family 

 didemnids. Relative to the specificity of the structures in 

 holothurians, we have this piece of significant information: 

 "These calcareous bodies are of great value to the system- 

 atist in classifying the smaller groups, such as genera and 

 species. Although their general characteristics are fairly 

 similar within the several families, the different shapes of 

 spicules are not sufficiently constant to be used as diagnos- 

 tic characters of such large divisions." ^^ In other words, 

 so far as these animals are concerned, should it be found. 



