86 SIDNEY F. HARMER. 
muscle. An alveolus which is incompletely roofed in is 
shown in fig. 6. 
The original extent of the alveoli is indicated by the grooves, 
which correspond with the original septa, on the surface of the 
ovicell (cf. figs. 7 and 14). It is obvious that the ovicell is 
gradually increased in size by the addition of fresh alveoli all 
round its margin. The vertical septa are absorbed after the 
alveolus has become part of the ovicell, and a continuous space 
thus results, through which the more centrally placed zocecia 
pass as free pillars. Fig. 6 illustrates this point, and the con- 
formation of that part of the roof of the ovicell which lies to 
the left of the right-hand zocecium indicates the loss of the 
vertical septa which were at first present. 
We have still to consider the earliest development of the 
ovicell, which, to the best of my belief, has not been studied in 
detail by any previous observer. Fig. 12, though by no means 
an early stage, will serve to introduce us to a phenomenon 
which is of the highest importance for the understanding of 
the history of the colony. This phenomenon may be spoken 
of as the occlusion of the fertile zoecium. 
In fig. 12, all parts of the ovicell which have received a 
complete roof are shaded. The preparation is a transparent 
one, made in Canada balsam, so that the walls of the zowcia 
can be seen through the roof of the ovicell. Parts which are 
seen through some other part of the colony are indicated in 
dotted lines. The colony is left-handed—z! and z? project in 
the ordinary way beyond the roof of the ovicell, while z?, the 
occluded zocecium, is completely closed by the roof, and would 
probably not have been visible at all in a dried, opaque pre- 
paration. In the immediate neighbourhood of the occluded 
zocecium arises the trumpet-shaped aperture of the ovicell. The 
study of decalcified sections is necessary in order to understand 
the significance of these facts, but it must for the present be 
assumed that we are justified in alluding to z? in this colony as 
the “fertile? zocecium—i.e. as the zocecium in which the 
embryo is first developed. 
The roofing in of the ovicell commences with the 
