258 THE HYDRA 



and 126) until they reach the places where they are used, upon 

 tentacles or hypostome. Arriving at this region, they become 

 attached to the supporting lamella and grow upward to pierce 

 the "roof" of ectodermal cells (cf. Figs. 122 and 124), thus becom- 

 ing exposed at the surface and finally developing their cnidocils in 

 the manner previously indicated. In the hydroid Tubularia it 

 seems that the cnidoblasts, after forming their nematocysts in the 

 stalk of the hydroid, may even migrate through the supporting 

 lamella and cndoderm into the enteron, in which they are carried 

 to the region of the tentacles. Here they reenter the endoderm and 

 again pass through the supporting lamella to reach the ectoderm 

 once more. In this layer they then migrate to their final position 

 on the tentacles. It is impossible to observe such migration in 

 hydra because the body is mobile and not enclosed by a skeleton- 

 like perisarc as is the body of the hydroid, where the cnidoblasts 

 can be observed to move past fixed points. But the lack of 

 developmental stages on the tentacles of hydra where the nemato- 

 cysts are used and the abundance of such stages on the body so 

 parallels the conditions observed in hydroids, that there can be 

 little question as to the similar conditions of origin and migration. 

 There is no evidence that the path of migration is so complex as in 

 Tubularia, but there is little doubt that nematocysts originate 

 in the body ectoderm of hydra and are carried by the amoeboid 

 movements of their cnidoblast cells to their place of use upon the 

 tentacles. 



The justification for this extended account of these microscopic 

 structures centers about the fact that so minute a " harpoon- 

 gun " can be produced and function, not as a hving cell but as a 

 non-living mechanism produced within a cell. The nematocyst 

 is capable of being discharged by the action of its cnidoblast cell, 

 or it may be discharged by external agencies, days after the cap- 

 sule has been separated from its cnidoblast. It may even be 

 appropriated by another animal and put to the use that would 

 have been made of it by the hydra. Just as we are awed by the 

 unmensity of space, as revealed by the telescope, so we are amazed 

 by the minuteness of such a mechanism as the nematocyst. If one 

 may modify the German of Rosel von Rosenhof, a student of 

 microscopic life in the early eighteenth century, 



Lies dieses Buch, und lern dabey, 



Wie gros Natur im kleinem sey. *. 



