Plate IV. 



Fig. 58. A portion of the ectoderm taken from the right side of a Pluteus in its first stage, 

 showing two different kinds of cells. The calcareous deposit of the right side appears 

 through the ectoderm. Treated with nitrate of silver. 

 B 59. Section of the ectoderm of a Pluteus six days old. Hardened in osmic acid. 

 » 60. Optical section of the ectoderm, drawn from a living specimen six days old. 

 B 61. Section of the eotodermic cells of the ciliated band. Hardened in osmic acid and stained 



with hsematoxylin. 

 B 62. Side view of an arm of a young Pluteus to show the relation in size between the large 

 flat ectodermic cells and the cylindrical bottle-shaped ones of the ciliated band. Treated 

 with nitrate of silver. 

 The same cells as those drawn in figure 61, though under a higher power. 

 Section of the body-wall of a Blastula. 



Portions of the two deposits of one side of the mature Pluteus. 

 -68, Mesenchyme cells of a Pluteus, about fifty hours old. Treated with osmic acid. 

 Calciferous cells which have deposited a minute star, drawn from a living Blastula. Dia- 

 meter of the cells: 0,oi — 0,oo9 mm. Size of the star; 0.oo4 mm. Upper view. 

 The same in a somewhat older Blastula. 

 -73. Pigment cells lying close inside the ectoderm; drawn from a living Pluteus. 

 -75. Centres of calcification with the tetrahedron in situ. Hardened in a very diluted so- 

 lution of osmic acid. In fig. 75 the tetrahedron is surrounded by a lump of plasm. 



76. Calciferous cells varying in size from O.ois mm., to 0,036 mm. Treated with osmic acid. 



77. Optical section of the body-wall of a Blastula showing the three-armed star situated be- 

 tween the blastoderm and the calciferous cells. Drawn from a living specimen. 



78 — 80. Three developmental stages of a calcareous spicule showing the relation between the 



deposit itself and the calciferous cells. Treated with a very diluted solution of 



osmic acid. 



81. Optical section of the body-wall at the centre of calcification showing the common pseu- 



dopodial plasm of the calciferous cells running close inside the Blastoderm. A very 



minute deposit has just begun to arise. Hardened in a diluted solution of osmic acid. 



