THE BRYOPHYTA 117 



the archegonia thus come to be seated on a steep 

 slope, facing towards the growing-point. In the mean 

 time the thallus goes on growing below the thickened 

 part, forming a thin membrane, while simultaneously a 

 membranous outgrowth arises above, behind the arche- 

 gonia, and completely overlaps the whole group, which 

 thus appears to be enclosed in a kind of pocket on the 

 upper surface of the thallus. This pocket is called the 

 involucre. The development of the involucre varies 

 much according to the position in which the plant 

 grows ; in dry habitats it reaches a great length, while 

 in wet places it remains short. 



We will now follow the development of the arche- 

 gonium itself. Like the antheridium, it arises from a 

 single superficial cell. This grows out and cuts off a 

 basal cell by a transverse wall. From the upper cell 

 the archegonium itself is developed. Three vertical 

 walls are first formed, separating a central cell from 

 three peripheral cells. A transverse wall cuts off a 

 cap-cell from the top of the central cell. Then the 

 peripheral cells, or some of them, divide vertically, so 

 that we have a ring of about five cells surrounding the 

 central one. Next, all the cells divide across by a 

 transverse wall, cutting the whole archegonium into two 

 halves, the lower being the venter and the upper the 

 neck (Fig. 54). The principal parts are now marked 

 out. The external cells of the lower ventral part grow 

 and divide, giving rise to a wall two cells thick, while 

 the central cell undergoes a single transverse division 

 into two very unequal halves; the small upper part 

 is the ventral canal-cell, the lower and larger cell be- 

 comes the ovum itself (Fig. 54). In the mean time 

 the neck elongates greatly, and all its cells divide 



