E antes . — The Morphology of A gat his australis . 13 
tubercle-like swellings on the surface, and even projecting almost free from 
the cytoplasm. A period of rest from mitosis then intervenes, during which 
a rearrangement of the nuclei and cytoplasm ensues. Even while the last 
divisions are occurring some beginning of this is made. A thin layer of 
cytoplasm— definitely distinguishable from that of the globose mass — forms 
over the upper portion of the proembryo. Whether this is secured from 
the cytoplasm of the egg, which is being constantly absorbed by the 
growing sporophyte, or is withdrawn from among the nuclei of the pro- 
embryo, could not be ascertained. As the nuclei begin to take definite 
position this cap increases greatly in thickness, and extends down over the 
upper half or two-thirds of the rounded mass. A somewhat similar region 
is set off on the opposite side — but within the original sphere — by a nuclear 
retreat. These two features are well shown in Fig. 28. A median trans- 
verse section of the proembryo at just this stage is shown in Fig. 23. It is 
evident that the cap of dense cytoplasm extends completely around the 
upper portion of the proembryo. Meanwhile enlargement occurs ; the 
spherical form is lost, and the mass becomes broadly top-shaped, the cap 
increasing this appearance. Orientation at this time is by no means 
constant. The proembryo points usually slightly obliquely or straight 
downward, but the apex may be turned at right angles to that. The nuclei 
group themselves in three tiers, — the first evidence of those into which the 
mature proembryo is so sharply differentiated. In their new position they 
become somewhat flattened and angular by mutual compression. The 
nucleoli become two to several. Fig. 24 shows all these features clearly. 
The three tiers are recognizable : the uppermost, a cap of a single layer of 
large, radially elongated nuclei ; the median, a cluster of a few smaller 
rounded nuclei ; and the apical, another cap of elongated nuclei, smaller 
than those on the opposite side. The size, shape, and position of the cells 
soon to be formed are foretold at this time by the nuclei. The upper 
peripheral nuclei, while maintaining their relative position, push slowly out 
into the cap of cytoplasm under which they lie, and the latter is finally 
entirely occupied by them, the line of separation from the section beneath 
being lost. 
The purpose of the cap clearly is to nourish the uppermost layer 
during its formation of large nuclei. The lesser zone of cytoplasm free of 
nuclei at the opposite pole, though of different derivation, also serves to 
build up a tier of nuclei on that side in a very similar manner. 
As soon as these processes are complete wall formation occurs. This 
first becomes evident by lines of demarcation in the cytoplasm, setting off 
portions about each nucleus (Fig. 24). Then delicate walls appear which 
become strong only at full maturity of the proembryo. 
The median group consists of eight to twelve cubical cells, small but 
rich in protoplasm, arranged for the most part in two tiers. In these no 
