REGENERATION AND DEATH 211 



mesenchyma, scattered about in which are free sim- 

 ple cells, 1 which are referred to by recent authors as 

 " parenchymal " or " formative " cells. Now if the 

 head or tail of a planarian be cut off, the part lost is 

 regenerated. The regeneration is effected not by the 

 growth of the old tissues, each producing of its own 

 kind, but chiefly (perhaps wholly) by the multiplica- 

 tion and differentiation of the " formative " cells which, 

 after migrating to where they are needed, produce 

 outer skin (epidermis), intestine, muscle, etc. They 

 constitute a store of undifferentiated cells, ready to 

 enter upon various active differentiations when occa- 

 sion arises. 



There is a marine animal called Ciona : it belongs 

 to the class of the Ascidians. Professor Jacques Loeb 2 

 discovered in 1892 that if the portion of the animal 

 containing the nerve ganglion, or rudimentary brain, 

 be cut out, it will be regenerated, and a new brain 

 formed. This discovery was confirmed by Pio Min- 

 gazzini, 3 a gifted Italian zoologist whose early death 

 we lament. Recently, L. S. Schultze 4 has shown 



1 So far as known to me these cells were first described by H. N. Moseley 

 (" On the Anatomy and Histology of the Landplanarians of Ceylon." Philos. 

 Transactions, 1874, p. 105). J. Keller in 1894 (Jeiia'ische Zeilschr. f. Niitur- 

 wiss., xxvii., 371-407) demonstrated their role in regeneration. Keller's inter- 

 pretation has been confirmed by W. C. Curtis (Proc. Boston Soc. Nat. History, 

 xxx., pp. 515-559) and Miss N. M. Stevens (Arch. f. Entwickehingsme- 

 chanik, xxiv., 350373, 1907). The cells are of the embryonic type, that is to 

 say, without any specialisation or differentiation. 



2 J. Loeb, Untersuchungen zur Phywlogischen Morphologic, 1891-92. 



3 P. Mingazzini, "Sulla regenerazione nei tunicati," Boll. Soc. Natur- 

 edisti, Napoli, v. (1891.) 



4 L. S. Schultze, " Die Regeneration des Ganglions bei Ciona intestinalis, L., 



