EARLY DEVELOPMENT OF A STARFISH 467 



in the echinoderms, often with two hydropores in certain species, 

 is the equivalent of the collar coelom. 



A word remains to be said regarding the apical plate. In 

 certain crinoids and echinoids it becomes a well developed organ 

 in fairly old larvae, in which definite ganglion cells appear. As 

 the adult nervous system arises from the ectoderm in close 

 proximity to the hydrocele, it would appear that these embryonic 

 nerve cells play no part in tKe process. Whether they degener- 

 ate or become mesenchyme remains undetermined. It is fairly 

 certain, however, that they furnish a strong bit of evidence in 

 favor of the theory that the apical plate of the trochophore 

 larva and of the echinoderm larva are homologous structures. 



In Pateria mineata it has been shown that many of the cells 

 of the apical plate completely lose their connection with the 

 ectoderm and, as mesenchyme, migrate through the blastocele 

 to the oesophageal section of the gut where they probably be- 

 come transformed into circular muscles. All signs that they 

 ever functioned as nervous elements appear to be wholly lost. 

 In this connection one calls to mind the work of Katschenko ('88), 

 Piatt ('94) and others who find that numerous cells leave the 

 neural crest in the head region of various vertebrate embryos and 

 become mesenchyme. It is possible that such cells represent 

 ganglionic elements in the primitive ancestor which formerly 

 innervated preotic structures now vanished or rudimentary. 

 The matter, however, remains unsettled for the vertebrates; such 

 is emphatically the case with the echinoderms where the species 

 studied are few indeed and the data far too scanty to enable one 

 to draw trustworthy conclusions. 



LITERATURE CITED 



Gemmill, J. F. 1914 The development and certain points in the adult structure 



of the starfish Asterias rubens. Phil. Trans. Royal Soc, v. 205, 



Series B. 

 Heider, K. 1909 Zur Entwicklung von Balanoglossus clavigerus. Zool. Anz., 



Bd. 34, 1909. 

 Katschenko, N. 1888 Zur Entwicklungsgeschichte des Selachierembryos. 



Anat. Anz., Bd 3. 

 Platt, J. B. 1894 Ontogenetische Differenzirung des Ectoderms in Necturus. 



Arch. mikr. Anat., Bd. 43. 



