THE GEEM-PLASM THEORY 



887 



invaginates into the interior of the hitherto single-layered blastula 

 (Fig. 92, A), some cells are separated off {M), and move independently, 

 constantly multiplying the while, into the clear gelatinous iiuid ( G) 

 which fills the cavity of the larva, and there they fix themselves, 

 some on the outer ectodermic layer, others to the various regions 

 and outgrowths of the archenteron {Ms). According as these cells 

 have established themselves at one or another point, they become 

 connective tissue, muscle, or skeleton cells of the dermis, or contribute 

 to the muscular layer of the food-canal and water-vascular system, 

 or, finally, become skeleton-forming cells of the calcareous ring which 

 surrounds the gullet of the sea-cucumber. In all this there is nothing 



Fig. 92. Echinoderm-larv«. A, blastula-stage; the primary mesoderm-cells 

 (Jf) are being formed at the subsequent invagination-area of the endoderm 

 (Ent). Ekt, the ectoderm. B, gastrula-stage ; the arehentevon {UD) has been 

 invaginated (EnJ), and between it and the ectoderm (Ekt) the mesoderm-cells 

 (Ms) migrate into the gelatinous fluid which fills this cavity. There they 

 attach themselves partly to the ectoderm, and partly to the endoderm. After 

 Selenka. 



to indicate a determination of the cells in one direction ; on the 

 contrary it seems as if the fate of the individual cells depended on 

 the chance conditions which may lead them to one place or to 



another. ' ; 



There are thus three possibilities of development, three kinds of 

 reaction, implied in these cells, which are all outwardly alike, and we 

 can only understand their r61e in the building up of this very 

 symmetrical animal if we assume that of these three only one is 

 in- each case liberated, by the specific stimulus exerted by the 

 immediate surroundings of the cellj so that it may become, according 

 to the chance position it takes up after its migratioii, either a skm- 

 cell, a muscle- cell, ■OT.a^ skeleton-forming cell. . ■ - 



