MATURATION OF OVUM AND DEVELOPMENT OF ALLOPORA. 591 
IV. The Formation of the Embryonic Ectoderm. 
At about the time that the first embryonic nuclei make their 
appearance the protoplasm becomes considerably vacuolated. 
The exact time and the extent of the vacuolation varies in the 
two species I have examined, and indeed in different individual 
eases of the same species. In very young embryos it is some- 
times difficult to distinguish true vacuoles from those artificially 
produced by the contraction of the protoplasm under treatment 
with reagents. I shall consequently lay but little stress in 
this paper on the time and mode of appearance of the vacuoles 
in Allopora; but I hope to treat this point more fully when I 
have had an opportunity of comparing my results with those 
obtained in working out the development of some other 
Hydrocorallines. 
At the stage represented in Pl. XXXVIII, fig. 17, the 
embryonic nuclei have increased enormously in numbers, and 
may be found scattered throughout the embryonic protoplasm. 
The vacuoles are numerous, especially in the distal hemisphere, 
and in some cases they are separated from one another by 
simple thread-like branching strands of protoplasm. The 
larger vacuoles are never quite superficial, but the periphery is 
bounded by a thin continuous membrane of protoplasm bearing 
a few very small yolk-spherules. Into the substance of this 
membrane some of the embryonic nuclei wander and take 
up their positions side by side (fig. 18). 
This peripheral membrane with the nuclei forms the first 
beginning of the embryonic ectoderm. It commences at the 
distal pole and gradually spreads all round the young embryo. 
When the nuclei have wandered into the peripheral membrane 
on the proximal side and formed there the first beginning of 
the ectoderm, cellular bodies of protoplasm, each containing a 
single nucleus, and together forming a well-defined, thick, 
columnar epithelium, have developed on the distal side. At 
the sides of the embryo the ectoderm is in a condition inter- 
mediate between these two extremes. It must be obvious, then, 
that in an embryo at this stage (Pl. XXXVIII, fig. 20) it is 
