THE BRYOPHYTA 



CAPSULE — 



MUSCI, THE MOSSES 



CALYPTRA 



CAPSULE 



447 



HEPATIC AE 



SETA- 



FOOT 



n H n 



? \ '/ 



A 



6 



D 



Fig. 437. — Comparison of the development of the sporogonium in the Hepaticae 

 (A, B and C) and in the Musci (D, E, F and G). {After Goebel.) 



of the Algae, and we will consider here one theory to account for their 

 evolution. According to this view both the antheridium and the arche- 

 gonium originated from gametangia of a type similar to those in Ectocarpus 

 (Fig. 438). It will be remembered that in this plant we saw that the game- 

 tangium consisted of a number of cells, each of which gave rise to a gamete. 

 It has been suggested that as a result of the migration on to land it was 

 necessary to protect the gametes from desiccation, and as a result the outer 

 layer of cells of the gametangium became sterile and functioned as a wall to 

 protect the cells inside, which produced the gametes. It will be seen that 

 by such a view it is comparatively easy to derive the antheridium. Here we 

 have a structure in which the outer layer of cells constitutes the wall, whilst 

 the cells of the interior divide up to form the antherozoids. The derivation 

 of the archegonium is more complex, but it has been suggested that after the 

 development of the superficial layer to form a wall, progressive sterilization 

 of the interior cells occurred, starting at the apex and proceeding backwards. 

 In this way it is explained that the neck canal cells were originally female 

 gametes, which first of all lost their walls and then their cytoplasm, till in 

 the typical archegonium they are represented by little more than nuclei. 

 The ventral canal cell is a sister cell of the oosphere, and hence we may say 

 that the ventral canal cell is a potential gamete, and in this connection it is 

 interesting to note that there are cases on record in which both the oosphere 

 and the ventral canal cell may be fertilized, and both of them commence to 

 develop embryos, though there is no case on record in which the embryo 



