ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 37 



interesting vital phenomena of the protoplasm became more fully 

 known. During the last thirty years, however, one discovery after 

 another has been made about the nucleus, the rssult of which is 

 that this neglected body has been shown to be of as much import- 

 ance to the elementary organism as the protoplasmic substance. 



It is of interest that the history of the nucleus is analogous 

 in some respects to that of the cell. The nucleus was also con- 

 sidered at first to consist of a vesicle; indeed, it was even held to be 

 a smaller cell inside the larger one. But just as it came to be 

 recognised that the protoplasm is the vital substance of the cell, 

 so by degrees it came to be seen that the form of the nucleus is of 

 minor importance, and that its vitality depends far more upon the 

 presence in it of certain substances, the arrangement of which may 

 vary very considerably according as to whether the nucleus is in 

 an active or a passive condition. 



Richard Hertwig (II. 18) was the first to enunciate this 

 clearly in a short paper entitled, " Beitrage zu einer einheitlichen 

 Auffassung der verschiedenen Kernformen," in the following 

 words : " It is necessary to state at the commencement of my 

 observations, as the most important point to be considered in 

 classifying the various nuclear forms, that they all possess 

 a certain uniformity in composition. Whether the nuclei of 

 animals, plants, or Protista be under examination, it is invariably 

 seen that they are composed of a larger or smaller quantity of a 

 material which, like the earlier writers, I shall call nuclear 

 substance (nuclein). We must commence with the properties of 

 this substance in the same way as he who wishes to describe the 

 important characteristics of the cell must begin with the cell 

 substance, i.e. protoplasm." 



Hence the nucleus is now defined, not, according to Schleiden 

 and Schwann's idea, as a vesicle in the cell, but as a portion of a 

 special substance which is distinct from the protoplasm, and to a 

 certain extent separate from it, and which may vary considerably, as 

 to form, both in the resting and in the actively dividing condition. 



We will now consider the form, the size, and the number of 

 nuclei in a cell, and then the substances contained in the nucleus, 

 and their various modes of arrangement (the structure of the 

 nucleus) . 



a. The form, size and number of Nuclei. As a rule the 

 nucleus in plant and animal-cells appears as a round or oval body 

 (Figs. 1, 2, 6, 16), situated in the middle of the cell. Since it is 



