774 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
certainly exists of the filaments or septa containing elements derived from the 
ectoderm. This must be borne in mind in considering the origin of the sexual 
elements which subsequently make their appearance in the walls of the dorso-lateral 
and ventro-lateral septa ; for their derivation from the ectoderm is brought within the 
bounds of possibility. All writers agree that both ova and spermatozoa in the 
Anthozoa are derived from entoderm, and this has usually been regarded as beyond 
all question. The probabilities certainly appear to be very strongly in favour of this 
view, but it must, I think, be admitted that the possibility of an ectodermic origin for 
the sexual elements is not entirely excluded. [See Appendix. | 
§ 11. Changes of external form, appearance of the tentacles, and general histological 
changes. 
When the larva abandons its free-swimming life and settles upon the bottom it has 
a more or less elongated form, and the posterior part of the body is very extensible 
and changeable in shape. The ectoderm and entoderm have undergone little change. 
The entoderm cells are large and clear, with scanty coarse granules, and very distinct 
large oval nuclei. The ectoderm cells still retain their high columnar form, and have 
a finely granular contents, which stains slightly. The cells are planted on the lamella, 
and many of them extend through the entire thickness of the ectoderm. Besides the 
columnar form, there are other more or less rounded cells in the deeper layers of the 
ectoderm. 
The body now elongates rapidly (figs. 176-178), and the ectoderm undergoes a 
great change. The columnar cells lose their form, become rounded, proliferate rapidly, 
and lose their connexion with the lamella. The outermost cells finally become 
flattened or fusiform, and form a thin layer covering the exterior of the body (see 
figs. 173-175). At the same time a considerable amount of clear gelatinous matter 
is formed, which sometimes entirely separates the cells from each other (figs. 160, 161), 
and forms the greater part of the ectoderm. In the deeper parts of the ectoderm 
appear rounded cells of various forms. Here and there are very large, deeply- 
stained, oval cells (fig. 166, sp.); others are nearly spherical, and groups of four 
deeply-stained cells are occasionally seen (fig. 166). Besides these, long fusiform cells 
may in some places be seen lying on the outer side of the lamella. These characters 
appear especially in the middle and posterior parts of the body. In the anterior 
region transitional forms may be seen, and at the extreme anterior end the columnar 
form is long retained. 
The entoderm cells undergo meanwhile little change; they vary greatly in 
appearance according to the state of contraction of the body. During contraction 
they are of a high columnar form but when the body is fully extended they become 
much shortened or even flattened. 
Leptogorgia presents the same general histological characters at this stage but the 
