28 GENERAL ANATOMY OF THE TISSUES. 



observations upon normal and pathological products have sup- 

 ported this view, and at the present time the formation of a 

 cell-membrane immediately around the nucleus requires de- 

 monstration, 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 common] v as 

 " cell development around portions of contents," more rarely 

 (in the embryo sac) by free development within parent cells.] 1 



§ 12. 

 A multiplication of cells by division certainly takes place in 

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

 Fig. 8. and in the first colourless blood-cor- 



puscles of the Tadpole (Remak). It 

 takes place also, in all probability, in 

 the colourless blood-corpuscles of em- 

 bryos, and in the chyle-corpuscles of 

 adult Mammalia under certain cir- 

 cumstances. In all these cases, we see, in elongating cells, 

 the production of two nuclei from the originally simple 

 nucleus, apparently by division ; the cells then suffer con- 

 striction in the middle, and contract more and more around 

 the nuclei as they recede from each other, and at last separate 

 into two cells, each of which contains a nucleus. In the 

 chick we see blood-corpuscles in all conceivable stages of 

 separation, so that at length they are connected only by a 

 delicate thread, and there can here be no doubt whatever as to 

 the actual occurrence of this process. 



Fig. 8. Dividing blood-corpuscles of the chick, x 350. 



1 [The endogenous development of secondary " nuclei" seems to us to be ex- 

 tremely doubtful, even upon tbe evidence adduced, and we have been unable to 

 observe anything indicating that regularity of occurrence and importance of function 

 attributed to the nucleolus by Professor Kolliker. In cartilage, in the tooth-pulp, 

 in the homogeneous layers of the cutis, 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 



