456 EMBRYOLOGY 



through the middle and produces two parts, which again grow into new 

 individuals (Kowalevsky, Simroth). Each part acquires a new mouth 

 and the complete organization of the normal animal. A division of this 

 kind, such as is found, for example, in Asteracanthion tenuupinus and 

 Ophiactis virens, may be called schizogony. 



An interesting form of regeneration, which can scarcely be called by 

 that name, since it transcends the meaning of that word and appears to 

 approach non-sexual reproduction, is described by P. und F. Sarasin 

 (No. 46). They saw new arms budding forth on the stump of an arm of 

 Linckia multifora in such a way as to seem to form a new star, which 

 remained attached to the stump of the arm (Fig. 225). P. und F. Sarasin 

 look upon this process as a budding of the starfish, and it is not to be 

 denied that it would merit this term, if a mouth-opening could be demon- 

 strated on the young starfish. 



General Considerations. — The development of the 

 varions divisions of Echinoderms offers many common 

 features, in which a close relationship of the five divisions 

 can be recognized. We saw that cleavage was always total, 

 tolerably equal, and its result a ciliated blastula. A typical 

 invagination gastrula arises from it. The mesenchyma cells 

 detach themselves from the invaginated part {Antedon, Astro- 

 pecten, Synapta). The fact that the development of the 

 mesenchyma in other forms {Echinoidea, Ophiuroidea) takes 

 place even before gastrulation is completed does not seem 

 to constitute an important distinction, because in such cases 

 it is from the same region — namely, the entodermal part of 

 the blastula — that the detachment of the mesenchyma cells 

 takes place, and because in another case (Holothuria) this 

 occurs during the beginning of gastrulation. The mesen- 

 chyma is doubtless to be traced to the same origin as the 

 mesodermal structures, which as coelomic sacs detach them- 

 selves from the archenteron. This assumption gains in pro- 

 bability from the fact that in a later stage of development 

 another separation of cells takes place from the epithelium 

 of the entero-hydrocoele. Enterocoele and hydrocoele are 

 the derivatives of the two coelomic sacs, which are constricted 

 off together from the apex of the archenteron. They then 

 separate into the two enterocoeles and the hydrocoele, which 

 latter enters into communication with the outside world by 

 means of the dorsal pore. 



