THE GERM-PLASM THEORY 



invaginates into the interior of the hitherto single-layered b 

 ^.92,^ some cells are separated off W , and. iZ 



7 t n S Z pying the whilej nit " ll '"•"• ^-<>J ^d 



which fills the cavzty of the larva, and there they ,i, ,1,, , 



some on the outer eetodermic lay,, others to the ,„i.„, Z^ 

 and outgrow hs of the archenteron [Ms), According as tl ",., N 



have estabhshed themselves at o, • another |lllillt . & ^ 



connective tissue, muscle, or skeleton ells 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. I,, all this there l nothing 



Ent 



Fig. 92. Echinoderm-larvae. A, blastula-stage; the primary mesoderm-oeUa 

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

 (Ent). Ekt, the ectoderm. B, gastrula-stage ; the archenteron UL has 

 invaginated (Ent), and between it and the ectoderm (Ekt the mesoderm-, i lis 

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

 attach themselves partly to the ectoderm, and partly to the endodi rm. A: 

 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 01 

 another. 



There are thus three possibilities of development, three kinds of 

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

 can only understand their role 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 cell, so that it may become, according 

 to the chance position it takes up after its migration, either a skin- 

 cell, a muscle-cell, or a skeleton-forming cell. 



