THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 195 



the spore coats begin to differentiate. The exospore first appears, and 

 just outside of it the rough perispore soon begins to develop. Then a 

 second layer, the mesospore, is formed within the exospore. Between the 

 protoplast, which is at this time very small, and the mesospore, and 

 between the exospore and the mesospore, there are developed two cavities 

 filled with a sporangial fluid which furnishes material to the growing 

 coats. Emphasis is placed on the fact that the coats are able to increase 

 in thickness while they have no immediate contact with the protoplast. 

 The protoplast now expands, after which a third coat, the endospore, is 

 formed at its surface. The mature spore thus has three coats according 

 to Fitting's interpretation, which Denke (1902) and Campbell (1902) 

 confirmed. 



e x. &m, e n. 



FIG. 69. The developing megaspore coat of Selaginella rupestris: 

 p, protoplast with nucleus; en, endospore; s.m., undifferentiated portion of "spore 

 membrane;" ex, exospore: the outer denser portion is the "perinium." (After Lyon, 1905.) 



Miss Lyon found that the spore coats (S. rupestris} begin to differ- 

 entiate in the midst of the " spore membrane" ("special wall:" Fitting), 

 rather than at its inner surface as Fitting thought. The exospore 

 first appears as a double zone, the outer part of which becomes the per- 

 inium (perispore: Fitting) (Fig. 69). The small protoplast gradually 

 expands and pushes back the undifferentiated inner portion of the spore 

 membrane; and while it does so a second coat is formed at its surface 

 and becomes the endospore (mesospore: Fitting) which increases in thick- 

 ness by lamination. In another species (S. emiliana) the exospore and 

 endospore form simultaneously. Miss Lyon thus finds two coats rather 



