STRUCTURE OF THE NUCLEUS. 375 



is at once tinged by them. Upon the application of iodine it 

 becomes deeper brown-yellow than protoplasm, and this led 

 Hofmeister to the belief that it is richer in albuminoidal mat- 

 ters. 1 Its behavior with digestive fluid and other reagents indi- 

 cates that, like the nucleus in the animal kingdom, 2 it contains 

 a substance rich in phosphorus. 8 



996. The surface of the nucleus generally appears to be 

 firmer and more highly refringent than the interior mass, and in 

 these respects is like the superficial layer of protoplasm. Even 

 with low powers of the microscope and without reagents the 

 inner mass of the nucleus is often seen to be far from homo- 

 geneous, generally containing granules, which are sometimes 

 irregular, sometimes regular in form. When a single large 

 granule is present, it is known as the nucleolus ; when two or 

 more, the nucleoli. These vary widely in number, size, and 

 shape. Besides such granules, vacuoles are frequently present. 

 Upon the application of suitable staining agents, and by the 

 use of high powers, the nucleus, formerly thought to be nearly 

 homogeneous, is shown to be a basic substance possessing a 

 finely reticulated structure. At times the nucleus appears to 

 be simply dotted throughout with fine points. 



997. When the bodies which are associated with its basic sub- 

 stance are granular, they are distinct from each other ; but when 

 in the shape of rods, fibres, or delicate threads, they are usually 

 conjoined to form a sort of network, or so connected together 

 as to make a long thread which is tangled in a complicated man- 

 ner. The basic substance of the nucleus, less highly colored by 

 staining agents than the rest, has been called Achromatin ; while 

 the portions which take color readily are termed Chromatin by 

 Flemming, nuclein 4 by Strasburger. 



During cell-division these portions of the nucleus undergo 

 remarkable changes of shape and position, which, with the 

 changes observable in the nucleus as a whole, can be illustrated 

 by a few special cases taken from Strasburger's treatise, and 

 given in nearly his words. 



1 Hofmeister: Die Lehre von der Pflanzenzelle, 1867, pp. 78, 79. 



a Hoppe-Seyler: Physiologische Chemie, i. p. 84, which contains a good 

 account of the literature of the subject. 



8 Zacharias : Botanische Zeitung, 1881, p. 169. 



* The only objection to the term nuclein is its previous application to the 

 proximate chemical substance rich in phosphorus which, although a part of 

 the nucleus, is not proved to be identical with the part which receives colors 

 most deeply. 



