34 Hjalmar Théel, 



VII. 

 The formation of calcareous deposits. 



The manner in which the calcareous deposits originate in the 

 Echinoderms, and the subsequent mode of increase of the skeleton are 

 by no means clear and offer a great deal of interest. When studying 

 these processes, the investigator soon perceives that the calciferous 

 cells, though free in the blastocœl, act by no means without method, 

 and that they cooperate for a common goal as if they were conscious. 

 Moreover, the processes apparently remind us of those which go on 

 during the formation of bone and dentine in the vertebrated animals. 

 In the Echinoderms too we may with good reason speak of cells ho- 

 mologous to the osteoblasts and ostoclasts, the calcareous substance ha- 

 ving been formed by the agency of the calciferous cells and absorbed 

 again in case of exigency by others or possibly by the same cells. Thus, 

 the formation and absorption of calcareous substance is effected by me- 

 senchyme cells of the same shape, as far as is known, which is not 

 the case in the formation and absorption of bone, the osteoblasts being 

 different in shape from the ostoclasts, — but this difference does not 

 seem to be of importance, since it is supposed that the latter are de- 

 rived from the former. Besides, if we succeed in elucidating this pro- 

 cess in the Echinoderms, it will certainly throw light upon the way in 

 which the osteoblasts and odontoblasts take part in the ossification, 

 whether the peripheral part of them becomes directly converted into 

 osseous substance or whether the ground-substance of bone is formed 

 outside the cells in an intercellular substance. 



It is a known fact that the larval as well as post-larval skeleton 

 characteristic of most Echinoderms is formed by the agency of the me- 

 senchyme cells. In Echinocyamus, in which the formation of calcareous 

 deposits begins eai'lier than in most other Echinoderms, the first ente- 

 ring cells move to two fixed places in the Blastula, arranging themselves 

 in two bilaterally symmetrical heaps or bands, one on the right and one 

 on the left side of the larva. It is a sight of the greatest interest to 

 follow these cells, to see how they move towards these two places as by 

 word of command in order to form there the two first centres of cal- 

 cification. As far as I remember I have never seen the formation of 

 calcai'eous substance take place before these centres consist of at least 

 three cells which contain one or more minute crystals apparently of an 

 indefinite shape. 



