ECHINO.CYAMUS PUSILLUS. 33 



dififerent developmental stages, one is soon convinced of this fact, PI. 

 Ill, fig. 50 — 57. As far as I know, Prouho*) alone seems to have ob- 

 served something similar in Dorocidaris papillata. Concerning the time 

 when the mesenchyme originates in the Echinoderms, the following opi- 

 nion prevails. In Echinoids, Ophiurids and Cucumaria (planci) the me- 

 senchyme arises earliest, i. e. during the blastula stage, and in Holo- 

 thuria (tubulosa) it makes its appearance with the first indication of 

 the archenteron. In Asteroids, Crinoids and Synapta on the contrary 

 the mesenchyme cells take their origin from the end of the fully grown 

 archenteron. If we take this for granted, Dorocidaris and Echinocyamus 

 should make an interesting exception, the wandering cells being loosened 

 from the entoderm during the whole process of invagination from the 

 blastula stage to the full- sized Gastrula. 



I may be allowed here to give in a few words my own opinion 

 with regard to the origin of the mesenchyme in the Echinoderms, which 

 is founded mainly on my investigations of Echinocyamus. 



1) With very few exceptions, the mesenchyme originates from the 

 entoderm alone and by means of immigration, and further investi- 

 gations will probably prove that the formation of mesenchyme in 

 many Echinoderms goes on not only during the blastula stage but 

 uninterruptedly also during the whole gastrulation. Of course 

 this is not the case in the Asteroids, Crinoids and Synapta, where 

 the immigration only commences when the archenteron is almost 

 grown. With regard to its origin by means of delamination, as 

 is supposed to be the case in Amphiura squamata and Psolinus 

 brevis, we seem to need more detailed accounts, before it is 

 taken for granted. 



2) The mesenchyme arises earliest, with very few exceptions, in 

 those groups of Echinoderms, in which the larvœ have the ske- 

 leton earliest developed. Most of the first entering cells are 

 calciferous. 



3) No bilaterally arranged arche-mesenchyme cells are to be detected. 



4) No true mesenchyme bands, homologous to the mesodermic bands 

 in Annelids, Mollusks etc., are met with in the Echinoderms, but 

 often, especially in the Echinoids, the first wandering cells which 

 enter, arrange themselves in two bilateral heaps or bands. 



1) Recherches sur le Dorocidaris papillata et quelques autres Echinides de la 

 Méditerranée. 1887. p. 344. 



Nova Acta Eeg. Soc. Se. ups. Ser. III. 5 



