XI.] OKGANIC DEVELOPMENT. 127 



gelatinous matter is emptied out of the sac, it will form a 

 new sac for itself ; bu.t the sac will not form any germinal 

 matter, nor will it manifest any vital properties. Here is 

 the first and simplest distinction of germinal matter and 

 formed material ; and we see that the germinal matter will 

 produce formed material, but the formed material will not 

 produce germinal matter. 



Formed material, however, is also produced in another 

 way ; not by separation from the germinal matter, as I 

 have just described in gromia, but by gradual transforma- 

 tion of the germinal matter itself into formed material. It 

 is in this manner that a fibrous structure, a kind of rudi- 

 mentary muscular tissue, has been observed to originate in 

 the larvae (or, to speak more correctly, the free swimming 

 embryos) of the Echinoderms ;^ and I think it likely, though Larva? of 

 I have not met with the remark, and do not speak with any Echmo- 

 confidence, that this mode of development is characteristic 

 of muscular and other characteristically animal tissues; 

 while development by means of the separation of the 

 formed material from the germinal matter, as in the 

 gromia, is characteristic of vegetables, and of those animal 

 tissues in which vegetative or nutritive functions are 

 carried on. 



When the formed material is separated from the germinal 

 matter, the separation takes place on the outside of the 

 mass of germinal matter ; and this gives origin to cells, of Cells, 

 which the inside consists of the soft, half-liquid germinal 

 matter, and the outside of harder formed material. The 

 gromia, described above, is an auimal consisting of a single 

 cell ; and there are whole tribes of vegetables that consist of 

 the same, especially the Diatomacese and the Desmideee, 

 which are classed as very simple forms of Algae. A great 

 part of the tissues of the higher animals and vegetables Cellular 

 consists of such cells, variously modified. So common, *^®®^^- 

 indeed, is this simple formation, that the cell used to be 

 regarded as the primary element of all organized tissue 

 whatever ; but the observations on the larvae of the 



' Professor Wyville Thomson on the Embryology of the Echinodermata, 

 Natural History Review, October 1 864. 



