Evidence from Somatic Histogenesis 



47 



indirect evidence whicli bears on the point favors over- 

 wlielmingly the view that the cytoplasm is chiefly responsible 

 for the shield. Huntsman supposes the sj)inules to be de- 

 rived from the cells of the celhilose tunic characteristic of 

 ascidians. He may be right in this; but it may be, too, tljat 

 they are derived from the epithelial lining of tlie sipluni. 

 The matrix of the cellulose tunic is undoubtedly largely if 

 not wholly produced by the ectodermal cells. It is usually 

 held to be secreted by these cells ; but in some cases, in 



FIGURE 47. SPIXULK C'EIJ. OF S'l'YEI.A MOXTEREYEXSTS (aFTER RFTTER AND 



FORSYTH ) . 



Perophora for example, a portion of the cytoplasm of the 

 cells seems to become transformed into the cellulose. The 

 process of transformation can be particularly well seen in 

 the cells which line the branchial siphon of developing bhis- 

 tozooids as shown in figure 32, plate III of my memoir.' - 

 The ])arts of the cell-bodies turned toward the cellulose are 

 here long drawn out and the protoplasm gradually becomes 

 indistinguishable from the surrounding cellulose substance 

 in which it is imbedded. If now we imagine this proto})lasm 

 to transform not into the characteristic cartilage-like cellu- 

 lose mass spread over nearl}'^ the whole surface of the body, 

 that from innumerable cells fusing into a connnon mass, hut 

 each cell to retain its individuality, its protoplasm becoming 

 the shield of a spinule, we should have what these styelids 



