THE VASCULAR CRYPTOGAMS 65 



casual phenomenon even in the homosporous Ferns. 

 Small and ill-nourished prothalli suffice for the pro- 

 duction of antheridia, which quickly fulfil their function 

 and make no great demands on the food supply. On 

 the other hand, archegonia are useless unless provision 

 be made for the nutrition of the embryo after fertilisation ; 

 and so we find female organs on full-grown and well- 

 nourished prothalli only. In the heterosporous Crypto- 

 gams provision is made beforehand, in the spore, for the 

 more abundant nutrition of the female prothallus. 



R DEVELOPMENT AND STRUCTURE OF THE SEXUAL ORGANS 



1. The Antheridia 



Each antheridium arises from a single cell, the upper 

 part of which grows slightly beyond the general surface 

 of the prothallus, and is cut off by a transverse wall. 

 It then undergoes a few divisions, so that the anther- 

 idium comes to consist of a central cell, surrounded 

 by two ring-shaped cells, one above the other, and 

 covered in on the top by a cap-cell. Some of the stages 

 of development are shown in Fig. 33, A. 



The central cell divides up repeatedly, and gives rise 

 to the spermatozoid mother-cells, the number of which in 

 each antheridium averages about twenty (see Fig. 33, B). 

 In each mother-cell one spermatozoid is formed. The 

 mature spermatozoid consists of a spirally coiled body 

 like a corkscrew, but thicker at one end than the other. 

 Near the thin end a number of excessively fine cilia 

 (contractile protoplasmic threads) are attached (see 

 Fig. 33, C). 



The development has been very exactly followed ; it is 

 known that the greater part of the body of the spermato- 

 5 



