54 GENERAL ANATOMY OF THE TISSUES. 



of the nuclei is preliminary to their breaking up into smaller cells. In 

 all these cases, it may be easily demonstrated that the nuclei multiply 

 spontaneously, but it is generally doubtful whether this happens by 

 division or by endogenous development. 



Cell-development round a mass of blastema containing a nucleus in 

 its interior, which may take place either freely or in an endogenous 

 manner (cell-development round portions of contents), had long ago 

 been seen by Von Siebold in the ova of Distoma globiporum, and by 

 Bergmann in the cleavage-masses of Rana, but no further importance 

 was attached to it. Vogt and Nageli were the first who regarded this 

 cell-development as a deviation from the theory of Schleiden and 

 Schwann, whereupon, supported by observations upon embryos, I (in 

 1844, " Entwick. d. Cephalopoden") placed this as a second kind of 

 cell-development under the name of " cell-development round investing 

 masses," beside that which takes place immediately round nuclei, and 

 pointed out its very extensive occurrence, especially in embryos, where 

 at first it is the sole mode. Later observations upon normal and patho- 

 logical products have supported this view, and at the present time the 

 formation of a cell-membrane immediately around the nucleus requires 

 demonstration, rather than the opposite method. Endogenous cell- 

 development occurs in many pathological products, most frequently in 

 cancer, yet the steps of the process have not yet been exactly made out. 

 In plants this mode of multiplication of cells is the most extensive, and 

 it occurs commonly as "cell-development around portions of contents," 

 more rarely (in the embryo sac) by free development within parent cells.* 



12. A multiplication of cells by division certainly takes place in 

 the red blood-corpuscles of the embryos of Birds and Mammalia, and 

 in the first colorless blood-corpuscles of the 

 Tadpole (Remak). It takes place also, in all 

 probability, in the colorless blood-corpuscles of 

 embryos, and in the chyle-corpuscles of adult 

 Mammalia under certain circumstances. In all 

 these cases, we see, in elongating cells, the pro- 

 duction of two nuclei from the originally simple 



FIG. 8. Dividing blood corpuscles of the chick; magnified 350 diam. 



* [The endogenous development of secondary " nuclei" seems to us to be extremely doubt- 

 ful, even upon the evidence adduced, and we have been unable to observe anything indi- 

 cating that regularity of occurrence and importance of function attributed to the nucleolus by 

 Professor Ktilliker. In cartilage, in the tooth-pulp, in the homogeneous layers of the cuti?, 

 and in other localities, in which unaltered " nuclei" occur, the presence and number of the 

 granules which might be called nucleoli is in the highest degree variable and uncertain. 

 The same irregularity as to the presence of nucleoli occurs in the plant (vide Von Mohl, 1. c., 

 and Schacht, " Die Pflanzenzelle," p. 30). 



Upon this subject consult the valuable memoir of Remak (Ueber extra-cellulare Enste- 

 hung Thierischen Zellen," &c., Mull. Archiv, 1852), which by no means deserves the epi- 

 thet of" no longer available ;'' in fact, Remak's views seem to be essentially correct. TRS.] 



