566 | General Notes. . [June, 
embraced by the four upper, smaller cells of the animal pole. By 
division of the lower cells there arises a third layer of four inter- 
mediate cells, and soon after these four others appear which are 
apparently derived from the four upper ones. Thereupon six more 
smaller cells are developed at the animal pole, and somewhat 
later eight more such appear at the vegetative pole, so that the 
embryo is now composed of thirty-six cells. At this stage the 
gastrula mouth begins to develop; at first, as a slight depression, 
which later becomes deeper, leading to the formation of a sym- 
metrical gastrula consisting of ecto- and endoderm. The two 
annuli or cycles of large cells, which represent the velum, are 
now differentiated 
The gastrula is next somewhat elongated, and near the blasto- 
pore an endodermal cell is pushed into the blastoccel to give rise 
to the mesoderm. The blastopore is soon displaced somewhat 
ventralwards, and simultaneously certain ectodermal cells are 
drawn inwards to form part of the wall of the cavity of the gastrula. 
There are two distinct, symmetrically disposed groups of mesoder- 
mal cells near the blastopore ; the largest of these cells still form 
e endoderm and take part in limiting the cavity of the 
gastrula. The blastopore is gradually shoved nearer to the velum, 
and in connection with it is developed, an cesophagus formed 0 
ectodermal cells. The mesodermal cells have multiplied, but re- 
tained their bilaterally symmetrical position. eae 
The cesophagus is now a spacious sac, from the posterior, 1n- 
ferior wall of which a radular sac has been invaginated. Imme- 
invagination, which Kowalevsky calls the pedal gland. Two 
longitudinal, anteriorly conjoined thickenings of the ectoderm, 
which encroach upon the mesoderm, form the rudiments of the 
edal and branchial nerves. The four nerve cords are gradually 
split off from the ectoderm and assume their definitive position i1 
the mesoderm. The cavity in the pedal gland becomes fill 
ectodermal cells support a tuft of cilia. At certain points, where 
n 
a clear vacuole. There now appear seven transverse furrows 0 
_ cellular body, enclosing a hollow cavity, and posteriorly 
“branchial ganglion appears as the widened ends of the 
of the segmental organs, vessels and sexual organs. Ata som” 
what later stage fibrils from multipolar cells are developed in the 
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with a slimy secretion. At the apex of the velar area a pair of- 
_ spicula: appear later, each spicule-forming ectodermal cell acquires — 
