JUNE 6, 1884. ] 
posterior border of the blastopore. During the clo- 
sure of the blastopore, which begins at the hind bor- 
der and progresses towards the anterior border, the 
pair of mesoderm cells pass inward, taking up a posi- 
tion between the ectoderm and the endoderm. The 
blastopore closes completely; but the last (anterior) 
portion to close corresponds in position with the in- 
ner opening of the oesophagus, which is an ectoder- 
mal invagination. 
Almost as soon as the gastrulation begins, the ecto- 
derm becomes clothed with cilia, which pass through 
the pores of the zona radiata, and are thus in a posi- 
tion to set the embryo, together with its envelope, in 
motion. 
One of the most interesting features of the develop- 
ment is the formation of an embryonic envelope at 
the expense of the ectoderm, only a portion of the 
ectoderm entering into the embryo proper. The 
ectodermal parts of the embryo arise from two sepa- 
rate portions of the ectoderm; viz., (1) a group of 
cells at the animal pole, and (2) the ectoderm cells 
of the blastoporicrim. The former, called the ‘ head- 
plate,’ bears a long tuft of cilia, and later gives rise to 
the supra-oesophageal ganglion: the latter, called the 
‘trunk-plate,’ forms the body, including the oral and 
post-oral region of the head. The rest of the ecto- 
derm is employed in forming the embryonic envelope, 
or serosa, which arises as a fold around the trunk- 
plate during the closing of the blastopore. The 
fold gradually closes up over the plate, and thus brings 
about conditions similar to those seen in insects. 
The inner layer of the fold, however, does not form 
an amnion, but is gradually absorbed in the trunk- 
plate. The head-plate stretches a short distance 
under the serosa, but is not completely enveloped. 
During the closure of the blastopore, the mesoblastic 
bands begin to form as two rows of cells budded off 
from the two mesoblasts, henceforth called ‘pole 
cells of the mesoderm.’ 
Very early in the gastrula stage a circular groove 
forms around the head-plate, thus forming a bound- 
ary-line between the plate and the serosa. This 
groove is formed by the retreating of the protoplasm 
of the cells in this region from the egg-membrane. 
In the same manner is formed a dorsal canal, leading 
out of the ring-canal, and stretching along the dorsal 
side, terminating in the free edge of the fold around 
the trunk-plate. As this fold closes up over the 
trunk-plate, and its inner layer is gradually taken up 
by the plate, there is formed a cavity between the 
plate and the outer layer of the fold, or the serosa. 
The ring-canal, the dorsal canal, and this cavity, form 
together a system of amniotic cavities. 
The main axis of the gastrula, which joins the ani- 
mal with the vegetative pole, does not coincide with 
the long axis of the larva. The animal pole of the 
gastrula axis corresponds to the anterior pole of 
the larva, but the vegetative pole is carried forward 
on the ventral side to a point about midway between 
the mouth and the hind end; and this bending of the 
gastrula axis brings the ‘ pole cells of the mesoderm’ 
exactly opposite the animal pole, at the posterior ex- 
tremity of the larva, This change in axial relations 
SCIENCE. 
685 
is brought about by the growth of the trunk-petal. 
That part of this plate which lies behind the pole 
cells shifts its position to the dorsal side, and thus 
pushes the pole cells forward. The extension of the 
trunk-plate on the dorsal side is soon followed by ex- 
pansion on the ventral side. This growth of the 
trunk-plate not only changes the axial relations, but 
also brings it into continuity with the head-plate, 
thus effecting a union of the two hitherto separate 
ectodermal portions of the embryo. 
The fully-formed embryo has a post-oral, ciliated 
band in the equatorial zone, close behind the mouth, 
and is attached to the egg-membrane and the serosa 
at the cephalic end. The escape of the embryo from 
the egg-membrane and the serosa requires several 
hours, and is attended with changes of the external 
form, especially in the region of the head-plate, that 
are not easily described without the aid of figures. 
It is the hind end of the body that first breaks through 
the envelopes. The rupture is made at the pole oppo- 
site the head-plate, and gradually increases in size 
as the body elongates, and forces its way outward. 
While the body is thus liberated, the head-plate is 
pulled away from the serosa and the membrane, but 
remains fastened at several points by string-like elon- 
gations of its substance. The serosa and membrane 
now form a sort of helmet, which the embryo bears 
about for some hours, and then throws off. 
Only two days and a half are consumed in the em- 
bryonic development, while the larval period, upon 
which the embryo now enters, and during which it 
leads a pelagic life, continues for an entire month. 
During this period the larva grows rapidly, without 
undergoing any important changes in form or in in- 
ner organization, except the formation of the ventral 
nerve-cord and the histological differentiation of the 
muscles. 
At the end of this period the larva is changed by a 
quick metamorphosis into the adult form. The 
metamorphosis is characterized by the loss of the 
ciliated band, the degeneration of the accessory or- 
gans of the oesophagus, the narrowing of the stomach, 
and the acquisition of the dorsal vessel. These 
changes seem to be correlated with the abandonment 
of a free pelagic life. The rapid growth does not 
take place uniformly in all parts, but most energeti- 
cally at the hind end of the body. This ‘ polarity of 
growth’ is characteristic of metameric and many 
non-metameric animals. 
At the beginning of the larval period no part of 
the nerve system is fully differentiated. During this 
period the ventral nerve-cord arises, not by the con- 
crescence of symmetrical halves, but as a single medi- 
an thickening of the ectoderm. The development 
and separation of this cord from the epithelium 
progresses from the anterior end of the body back- 
ward. The oesophageal commissures form later 
than the ventral cord; and their development pro- 
gresses in the opposite direction, beginning at the 
anterior end of the cord, and advancing towards the 
head-plate. Last of all arises the supra-oesophageal 
ganglion from the deeper cells of the head-plate, 
which bears the two pigment eye-spots. The com- 
