supporting lamella

HISTOLOGY AND DEVELOPMENT OP MYBIOTHELA PHBYGIA. 507
THE EARLY STAGES IN THE FORMATION OF THE GONOPHORE
AND THE ORIGIN OF THE SEXUAL ELEMENTS.
The points noticed-under this heading may be regarded as
an addendum to Korotneff's account of the development of the
gonophore (3) alluded to above. His brief and somewhat
diagrammatic account is illustrated by figures which unfortunately
show little attempt at histological detail.
The Structure of the Ectoderm of the Blastostyle.—In
each blastostyle two regions may be distinguished ;
a distal and shorter region bearing several small capitate tentacles,
and a proximal region, on the whole free from tentacles,
on which the gonophores are developed. The latter, embraces
two-thirds to four-fifths of the whole length of the blastostyle.
The ectoderm of the distal portion resembles that of
the body generally, with the exception that the muscle-fibres,
which form its deepest layer, are not so strongly developed.
Its structure is shown in PI. XXXVI, fig. 1. Like the ectoderm
of the animal generally, it is covered superficially by a wellmarked
cuticle, which, when stripped off and examined with a
high power, is seen to be divided into irregular areas, doubtless
corresponding to the subjacent cell layer. This latter is composed
of long columnar cells, tapering at their lower end. Each
cell is about 25 /* long, though their dimensions possibly vary
according as the whole blastostyle is in a condition of extension
or contraction. Lying here and there between the pointed
bases of these cells are scattered ganglion-cells, which are connected
with a rich plexus of nerve-fibrils situated in the deepest
part of the ectoderm and immediately over the muscle-fibres.
The latter run longitudinally, and immediately overlie the supporting
lamella. Fibrils from the basal nerve plexus pass
between the columnar cells towards the surface, and, in tangential
sections, are seen to form a superficial plexus between the
columnar cells.
The same type of nucleus is found throughout the whole
ectoderm. It is characterised by the presence of a distinct