14 



Transactions. 



in these eases is that of the lowest tier of neck-cells. In fig. 15 is shown 

 the head of a small limb of a prothallus with archegonia in different stages 

 of development, in surface view. 



There are not lacking signs of dorsiventralitv in the distribution of 

 the sexual organs, but these are probably unimportant. For example, 

 the old antheridia are sometimes much more numerous along the edges 

 of the prothallus (in the plane in which it naturally lies), and also at the 

 growing apices the young antheridia sometimes occur more numerously 

 towards the edges. This tendency to dorsiventralitv is more apparent 

 still in the fact that in some of the younger prothalli one surface was 



antb 



ax. 



Fig. 16. — Transverse section of limb of prothallus behind growing apex, 

 showing antheridia and archegonia. x 100. 



noticed to be almost if not entirely free from rhizoids and sexual organs, 

 whilst the opposite surface bore them both. In the young prothallus 

 shown in fig. 12 one surface was quite naked and smooth, but on the other 

 there were a fair number of rhizoids, and the surface was noticeably rough 

 on account of the protruding of the epidermal cells in a papillose manner, 

 and also at the edges were both rhizoids and antheridia to be seen. These 

 indications of dorsiventrality in the distribution of the sexual organs are 

 not, however, always to be observed, and, on the whole, both antheridia 

 and archegonia may be said to be distributed more or less evenly around 

 the surface. 



