218 Dr. J. E. Gray on tlie Arrangement 



The germination of the spores also gives some insight into the 

 origin of the spiral bands. 



When the spore breaks out, the contents form a coat uni- 

 formly spread over the wall, with only a slight indication of 

 spiral arrangement {a, b, fig. 1. PI. VIII.). As the young cell 

 grows, this becomes broken up, and the originally irregular and 

 imperfect slits thus produced, subsequently cut in a continuous 

 course through the originally uniform coat, which is now slit up 

 into regularly arranged bands (PI. VIII. figs. 2, 3). The cause 

 why the coating of the wall tears up into spiral and not recti- 

 linear bands, remains unknown here, just as in the origin of all 

 other spiral forms in the vegetable cell. Germinating plants of 

 Spirogyrce with only one spiral band, might, perhaps, give an 

 opportunity of discovering more accurate particulars of this pro- 

 cess. That the cytoblast — Meyen^s ' central-organ'— notwith- 

 standing the mucilaginous filaments running out from it to the 

 borders of the spiral bands — plays no part here, seems to me 

 so much the more probable, that I doubt its actual existence in 

 the spore and in the young unicellular plant. I never found 

 the cytoblasts in the spores, even when the contents were gently 

 pressed out, which would make it clearly visible, and just as little 

 could I detect it in the much more transparent young, unicellu- 

 lar plant (figs. 2, 3). It is first found in the two- celled plants, 

 and many-celled specimens have, one in each cell, even the radi- 

 cal cell; it is not oval, but round (fig. 1 c, m, m, m). Alex. 

 Braun has shown the part it plays in the formation of new cells 

 in the Spirogyra^. It appears, therefore, that it originates in 

 the unicellular plant immediately before the formation of the 

 septum, and then quickly causes the formation of two new cyto- 

 blasts, either through solution or subdivision, and thus we should 

 bring its presence in all cells of old plants into agreement with 

 its absence from the spores and unicellular plants. 



[To be continued.] 



XXI. — Revision of the Families of Nudibranch Mollusks, with 

 the description of a new Genus of Phyllidiadse. By J. E. Gray, 

 Ph.D., F.R.S., V.P.Z.S. &c. 



The very important results which were obtained by the examina- 

 tion of the tongues and teeth of the Ctenobranchous Mollusca, 

 which were partly published in the last Number, have induced 

 me to continue my researches on these organs in the Nudi- 

 branchiate Mollusca. They have resulted in two important facts : 



* hoc. cit. p. 257 et seq. 



