STUDIES ON THE DEVELOPMENT OP ECHINOIDEA. 
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became flattened on the vegetative pole and the luesenchyme 
cells wandei’ed inwards. The raajoi-ity of them formed a ring 
round the periphery of the flattened surface, but some 
wandered up the sides of the blastula and even reached the 
animal pole. At this pole there is a tuft of specially long 
cilia. The invagination which forms the archenteron began at 
the vegetative pole, and the ring of mesenchyme cells 
became thickened at two spots diametrically opposite to one 
another, and in these thickenings the rudiments ot the larval 
skeleton appeared as two trifid “stars.” Each arm of each 
star grew out, the growth being caused by the deposition of 
calcareous matter by the mesenchyme cells which cling to 
the arm. One arm of each “star” grows upwards and 
outwards — this is the rudiment of the post-oral rod of the 
skeleton ; one arm grows backwards towards the opening of 
the invagination or “ blastopore ” and is the rudiment of the 
future “body rod ” of the skeleton, and one grows hoi’izon- 
tally across and forms the “ hor i zo iital rod ” of the skeleton. 
The invagination now deepens and forms the archenteron 
and the larva is now termed a gastrula. From the apex of 
the archenteron a single transversely elongated vesicle is cut 
off, which is the rudiment of the coelom. This stage, 
reached in two and a half days, is represented in PI. 14, 
fig. 3. At the animal pole of the embryo a tuft of specially 
long cilia (cil.) is to be seen. 
In the course of the next day the stornodaeum makes its 
appearance as a shallow pit on one side of the anterior end of 
the larva. The side of the larva on which it appears becomes 
concave and constitutes the future ventral surface. The 
pit rapidly deepens and reaches the apex of the archenteron, 
with which it unites, and thus the alimentary canal is com- 
plete. The coelomic sac has just previously divided into right 
and left halves. From the “star” on each side another 
branch is given off which extends upwards at the sides of the 
stomodaeum and here causes a slight protrusion of the ecto- 
derm, so that the outline of the anterior part of the larva 
becomes quadrate instead of being rounded as it was pre- 
