DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
747 
subsequent history was not followed. It is very probable that in this case also the 
union was accidental and was produced by the crowding of the larvae in small aquaria. 
The formation of the septa and tentacles will be described in the following section, 
and we may now consider the internal histological changes which have been in pro¬ 
gress during the stages just described. 
At the close of segmentation, the embryo (fig. 97) is a solid planula consisting of a 
central mass of large rounded cells, enclosed by a layer of cuboidal ectoderm cells. 
As development proceeds, the cells of both layers continually decrease in size by 
multiplication, and those of the ectoderm gradually assume a marked columnar form. 
At the same time, the character of the cell-contents changes somewhat, the deutoplasm 
spheres disappearing from the ectoderm cells, which accordingly appear less coarsely 
granular, and remaining only in the central cells, where they continue to be very 
distinct, until a short time before the appearance of the digestive cavity. 
The structure of an embryo of the stage superficially shown by figs. 12 and 13, is well 
shown in fig. 118. The section figured is from Leptogorgia, chosen on account of its 
good state of preservation; but it agrees in nearly all respects with sections through 
the corresponding stage of Renilla. The outer envelope consists of a single layer of 
cuboidal cells, in many of which are visible large rounded nuclei. The cells are 
destitute of membranes. Their contents are granular, but destitute of distinct 
deutoplasm spheres, and are scarcely stained by the picrocarmine. The peripheral 
zone of earlier stages is not visible, but in Renilla sections of this stage, it appears 
very clearly, as shown in fig. 119, taken from a somewhat later stage. 
The central part of the embryo is occupied with a solid mass of large rounded 
entoderm cells, or, as they may for the present be called, central ceils. The latter are 
enclosed by delicate but distinct membranes, which separate them sharply from each 
other, and from the surrounding ectoderm. N uclei are visible in many of them ; and 
some of the larger ones, being in course of division, contain two nuclei. The character 
of the cell-contents varies somewhat in different parts of the central mass. The 
more centrally placed cells are closely packed with clear spherules of deutoplasm left 
unstained by the carmine, between which is a kind of network of finely granular, 
deeply stained matter. The nuclei appear as clear vesicles, surrounded by deeply 
stained, finely granular areas. Passing towards the outer portions of the central 
mass, the deutoplasm spheres become less numerous, disappearing almost entirely 
in the outermost cells which adjoin the ectoderm. 
It is important to notice this early differentiation in the distribution of the 
deutoplasm ; for it indicates either that the deutoplasm is more abundant here 
even in early stages, when no difference between the central and peripheral parts 
of the egg is apparent to the eye, or that the protoplasm of the outer portions is 
more active, and hence assimilates more rapidly the deutoplasm. Either alternative 
