296 WILSON. [VoL. IX. 
mass becomes more and more compact until the cells of which 
it is composed acquire an irregularly polygonal shape owing to 
mutual pressure (Fig. 25). The ectoderm cells covering the 
pole become more or less cubical, and at this time do not differ 
in appearance from the subjacent parenchyma. They neither 
develop pigment nor cilia, and this end of the embryo is there- 
fore sharply marked off from the rest of the body. The 
remaining part of the parenchyma is made up of amoeboid 
cells provided with slender processes connected together into 
a network. The bodies of all these cells are plump and stain 
well, When the embryo has reached this stage of development 
(Fig. 25) spicules make their first appearance. They are few 
in number and mostly the long, slender oxytylotes. Besides 
the oxytylotes some curved spicules appear, the embryonic 
representatives of the bow-shaped spicules shown in Fig. 3d. 
The development proceeds a little farther than the stage 
shown in Fig. 25, and the embryo is tnen set free as a ciliated 
larva which escapes from the body of the parent through an 
osculum. In Pl. XVI, Fig. 26, a surface figure of this larva is 
given, and in Pl. XVI, Fig. 29, a longitudinal section. The 
greater part of the body is of a deep orange color but the 
posterior pole (f..) is unpigmented. The line of separation 
between the two regions is a perfectly sharp one. The pos- 
terior pole ends in a pointed protuberance (Fig. 26) which 
appears to be a specific characteristic. A bundle of straight 
spicules (oxytylotes) is conspicuous in this end of the larva, 
In its general appearance and motion the larva is very like 
a coelenterate planula. Like the latter it may swim freely 
through the water, or may creep worm-like over the bottom 
and sides of the dish, the pigmented pole being posterior. 
The cells of the ciliated ectoderm are very long and slender 
and the nuclei are packed closely in several tiers, so as to form 
avery conspicuous zone in sections (Fig. 29). The arrange- 
ment of several ectoderm cells is shown in the maceration 
preparation, Pl. XVI, 4, Fig. 31, and one of the cells more highly 
magnified in a of same figure. There is a single flagellum to 
each cell. In the peripheral end of the cell is deposited the 
orange pigment, in the shape of small rounded masses (£.a.). 
