THE VASCULAR CRYPTOGAMS 



21 



divisions ; the small cells thus formed become rounded 

 and lie freely in the space enclosed by the external layer 

 (Fig. 12, C). Subsequently the external cells become 

 disorganised, their contents contributing to the nutrition 

 of the central group. The prothallus - cell, however, 

 persists all through. The round central cells are alone 

 concerned in the 

 further development. 

 When the external 

 layer becomes dis- 

 organised, its cell- walls 

 disappear and the 

 contents flow together 

 into a structureless 

 mass, in which the 

 Tound cells are em- 

 bedded. In the mean- 

 time important changes 



~sp. m, 



go on in the contents 

 of these cells. The 

 large nucleus, which 

 each cell contains, be- 

 comes converted into 

 a long, rather club- 



FIG. 12. Selaginella; germinating micro- 

 spores. A, young stage ; p, prothallus- 

 cell ; first wall formed in antheridium. 

 B, more advanced stage ; c, central cells 

 which will form spermatozoids. C, mature 

 stage ; sp. m, sperm atozoid mother-cells, 

 surrounded by cells of wall of anther- 

 idium. D, free spermatozoids, each with 

 two cilia. A, magnified 290 diameters ; 

 B, magnified 290 diameters ; C, magni- 

 fied 640 diameters; D, magnified 780 

 diameters. (After Belayeff.) 



shaped body, which 



has a spiral twist. At the thin end of this body there 

 is a little protoplasm, and at this point two excessively 

 fine protoplasmic threads are attached (see Fig. 12, D). 

 The whole body now constitutes a spermatozoid, and the 

 protoplasmic threads are its cilia. The cell in which a 

 spermatozoid is formed is called its mother-cell. The 

 surrounding cells having completely broken down, the 

 spermatozoid mother-cells are let loose into the water, 



