ON THE DEVELOPMENT OF LICHENOPORA VERRUCARIA. 125 
is destined to form a single larva, is a small ovoid mass of cells 
consisting of a well-defined external layer and an equally 
clearly defined inner layer. One wall of the embryo becomes 
invaginated to form the larval “sucker ”’ (cf. 6, pl. xxiv, figs. 
22,23). This process is seen to be taking place in some of 
the embryos shown in fig. 11. 
It is not always easy to distinguish between cases in which 
the sucker is being invaginated and cases of embryonic fission. 
One difference may, however, be pointed out between the two 
processes. The invagination of the sucker never results in 
the division of the inner layer of cells into two separate por- 
tions, nor does this layer even appear divided in a section of 
the embryo. Fission, on the contrary, results in the complete 
separation of the inner layer into two halves, one of which 
belongs to each of the products of division. In some cases the 
ovicell contains larger and more irregular masses of embryonic 
tissue. These probably give rise to more than two secondary 
embryos. 
I have satisfied myself of the normal occurrence of the 
process of fission, as described above, in a number of colonies ; 
and I believe that the process continues, in healthy colonies, 
for a considerable time. I have not made preparations of 
colonies of all the larger sizes; and I am not able to say what 
is the upper limit of size reached during this stage. The 
largest colony among my sections, in which the embryonic 
structures are in Stage G, measures 1°60 mm. in diameter, 
and the smallest measures ‘80 mm. I have, however, found 
mature larve in somewhat abnormal colonies which measure 
only ‘56 mm. in diameter. The average diameter of fifteen 
colonies measured is ‘93 mm. 
Although the fertile brown body passes into the cavity of 
the ovicell during this stage, it is still possible to obtain satis- 
factory evidence of the identity of the fertile zocecium in well- 
orientated horizontal sections. If an ordinary zoccium loses 
its polypide without developing a new bud to take its place, 
the previous existence of the polypide is shown by the occur- 
rence of a brown body in the zocecium. There is no way by 
