54 THE VOYAGE OF H.M.S. CHALLENGER. 



The female gonopliores 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 ampullae and their contained ova and planulse remain until maturity 

 immersed in the coenosteum beneath its surface, in Errina the ampullae project more and 

 more above the surface as development proceeds. 



The spadix in Errina labiata is at first cup-shaped (PI. IV. S), 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 (PL 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 IV. S). 



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. E C). 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 (PL IV. B). 



In fine sections of mature planula? the fine structure of the ectoderm and endoderm 

 is well seen. The ectoderm forms a thick layer composed of alternately placed 

 transparent and opacpie 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 XL fig. 9, P»). 



