54 THE VOYAGE OF H.M.S. CHALLENGER. 
The female gonophores are closely similar in structure to those already described as 
occurring in Pliobothrus symmetricus ; but there is this great difference—that whilst in 
Pliobothrus the ampullze and their contained ova and planule remain until maturity 
immersed in the ccenosteum beneath its surface, in Errina the ampulle project more and 
more above the surface as development proceeds. 
The spadix in Errina labiata is at first cup-shaped (Pl. IV. §), the walls of the cup 
being composed of a very thick layer of endoderm. The cavity of the cup is directed 
towards the surface of the coral, and within it rests the single large ovum with its distinct 
germinal vesicle and spot. Each ampulla contains invariably only one spadix and ovum. 
The ovum is covered over in the cup by a reflection of the ectodermal investment of the 
spadix (PI. IX. fig. 4). 
The stages of yelk division were not detected. The ovum becomes developed into the 
condition of a planula within the ampulla. As development proceeds, the embryo 
becomes gradually greatly increased in size, and assumes a form corresponding to that of 
the containing ampulla already described. As the process proceeds the spadix becomes 
divided at its margin into a series of lobes, which lobes subdivide, branch, and unite to 
form a network, and encroach over the surface of the embryo until more than half of the 
proximal surface of the embryo becomes thus embraced by the reticulate cup of the 
spadix (Pl-IVs'8). 
The ectodermal layer of the embryo seems to be formed from the general mass by 
delamination. No trace of any process of invagination was observed ; but all stages were 
seen which would appear to prove that the ectoderm layer is gradually differentiated 
at the surface from the outer elements of the mass. The ectodermal layer when first 
observable as distinct, appears finely granular in structure, whilst the abundant 
endodermal mass is composed in large part of highly refracting oil-globules. The 
ectoderm, as development proceeds, shows a striation directed perpendicularly to the 
surface of the embryo all over, and this condition is very conspicuously marked in the 
fully-developed planula (Pl. IV. EC). The mature planula is elongate-ovoid in form, 
and is slightly folded once upon itself in order to accommodate itself to the confined 
space within the ampulla. The layer of ectoderm described as investing the surface 
of the ovum, and derived from the spadix, persists as a covering of the mature planula 
until set free (PJ. IV. B). 
In fine sections of mature planule the fine structure of the ectoderm and endoderm 
is well seen. The ectoderm forms a thick layer composed of alternately placed 
transparent and opaque tracts disposed vertically to the surface of the planula. The 
more opaque tracts contain numerous nuclei and thread cells in process of development. 
The dark tracts fuse together towards the inner region of the layer, and form a con- 
tinuous mass full of nuclei which rests upon the basement membrane, as yet little 
differentiated but still clearly indicated (Pl. XT. fig. 9, B). 
oF 
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