BELL-GASTRULA OF THE LANCELET. 445 



which immediately surrounds the intestinal cavity, is the 

 entoderm, the inner or vegetative germ-layer, from which 

 are developed the wall of the intestinal canal and all its 

 appendages (Fig. 155, 156, i). The outer cell-stratum, the 

 part of the blastula not inverted, is the exoderm, the outer 

 or animal germ-layer, which furnishes the rudiment of the 

 body-wall, the skin, the flesh, the central nervous system, 

 etc. (e}. The cells of the inner stratum, or entoderm, are 

 considerably larger, duller, darker, and more adipose than 

 those of the outer stratum, or exoderm, which are clearer', 

 brighter, and less rich in fatty particles. Thus, even during 

 the process of inversion, a differentiation takes place between 

 the inner inverted stratum and the outer uninverted. The 

 cells of the outer layer are soon covered with fine bright 

 hairs; fine, short, thread-like appendages, grow from the 

 protoplasm, which keep up a constant vibratory motion. 



Fig. 156. Gastrula of a Chalk-sponge (Olynthus) : A, from the outside; 

 B, in longitudinal section through the axis ; g, primitive intestine ; o, primi- 

 tive month i, intestinal-layer, or entoderm ; e, skin-layer, or exoderm. 



