398 WILSON. 
EXPLANATION OF PLATE XXI. 
(Zedania Brucez.) 
Fic. 75. Section of gemmule, in which the ectoderm layer is clearly differen- 
tiated. D.4. 
Fic. 76% Part of Fig.76; D4" 
Fic. 78. Longitudinal section through unpigmented pole of swimming larva, 
shortly after birth. D.4. 
Fic. 79. Surface view of larva just born. <A.4. 
Fic. 80. Surface view of older larva— unpigmented “plug” now conspicu- 
ous. <A.4. 
Fic. 81. Longitudinal section through swimming larva, a day old. C.4. 
Fic. 82. Attached larva with spicular pole pulledin. A.4. 
Fic. 83. Vertical section through sponge just attached. Ciliated ectoderm 
entirely metamorphosed, unpigmented pole still recognizable. C.4. 
Fic. 84. Surface view of recently attached sponge — spicular pole lost. A.4. 
Fic. 85. Surface view of sponge just attached — shows how spicular pole 
loses its identity — spicules are being distributed to various parts of sponge. A.4. 
Fic. 86. Surface view. Sponge is solid, and margin is thrown into irregular 
lobes (first stage in formation of ectodermal membrane). A.4. 
Fic. 87. Part of periphery of preceding figure. D.4. 
Fic. 89. Part of periphery of Fig. 88. D.4. 
