MUSEUM OF COMPARATIVE ZOOLOGY. 341 



p. Tissues. — Observations upon metamorphosed nuclei of tissice cells, 

 certainly referable to a process of division, are mostly of quite recent 

 date, and the papers which treat of them are largely the result of the 

 stimulus afforded by discoveries in connection with segmentation. 



The papers of Biitschli and Strasburger in 1875 served to recall the 

 attention of Mayzel ('75)* to certain appearances of nuclei which he 

 had often observed in his studies on the regeneration of epithelium, and 

 which up to this time had remained problematic to him. In essential 

 agreement with the observers alluded to, he at once came to the conclusion 

 that the existence of numerous coarse granules and fibrous structures 

 in the nuclei was connected with the process of nuclear division. 



The cornea of the rabbit and the cat, but more particularly the 

 cornea and other epidermal structures of the frog, were the objects 

 studied. Mayzel was unable to confirm by studies on fresh specimens 

 the results attained in the use of different reagents. Although occa- 

 sionally observed in regions of normally preserved epithelium, the 

 appearances were most frequently met with in the tracts where regen- 

 eration had followed an artificial removal of the epithelium, and then 

 not at the edge, but in the midst, of the regenerated portion, and in the 

 deep rather than in the superficial layers. He distinguished three 

 principal forms of the nucleus, without, however, being able to affirm 

 positively that they follow one another in the order in which they are 

 described : — 



(1.) Large oval nuclei of twice the diameter possessed by their 

 neighbors, either coarsely granular at the periphery only, — thus dis- 

 closing the nucleoli, — or throughout ; then such as have their granules 

 elongated into threads, and knotted together ; and finally those with 

 similar filaments, alike in thickness but of various lengths, arranged 

 radially about a central point. 



(2.) Large, elongated, spindle-shaped nuclei, with a thick transverse 

 disk which appeared either more nearly homogeneous, or else as if com- 

 posed of coarse, refringent granules of unequal size. Occasionally the 

 disk appeared double. The remnant of the indefinitely outlined nucleus 

 was delicately fibrous in the direction of the long axis, the fibres so 

 converging at the ends of the nucleus as to give it the appearance of two 

 fibrous cones placed base to base. 



* As I learn from Flemming ('75, p. 186) and Strasburger ('76, p. 230), Klebs 

 ('74) had already noticed early in 1874, in studying the regeneration of epithelium, 

 a radiate arrangement of the protoplasm which he connected, however, with the genesis 

 of new, rather than with the division of previously existing, nuclei. 



