III. 8 



CHEMICAL COMPOSITION 



139 



The gastrula wall is often smaller than the archenteron, and, 

 as the strength of the solution is increased, becomes still further 

 reduced, until nothing of it is left but a small button at the 



FIG. 73. ft. Larva with three skeletal spicules, and a ' cell-rosette ' at 

 the end of the archenteron. b. Larva with skeleton more than three 

 spicules - and arms developed. The neck is invaginated into the ecto- 

 derinal portion, the gut tripartite, c. Five-armed Pluteus with five 

 skeletal rods. The gut is normally invaginated and tripartite, d. Larva 

 of Echinus microtiiberculatus. There is a neck, and the gut is partly 

 invaginated. In the blastocoel are aggregations of inesenchyme and 

 pigment cells. (After Herbst, 1895.) 



animal pole, which only indicates its real character by the long 

 cilia which it carries. Such larvae Herbst terms ' Holoento- 

 blastia'. This nearly complete suppression of the ectodermal 



