262 Causes and Course of Organic Evolution 



But, while Spirogyra and some related simple conjugate 

 algae exhibit the above karyokinetic changes, most tissue cells 

 show rather different division phases, that require considera- 

 tion and comparison with Spirogyra. In these the chromatin 

 substance, Avhile still largely massed in the nucleolus as in 

 Spirogyra, gradually becomes more and more richly distributed 

 radially from the nucleolus so as to form a chromatin network 

 within the nucleus, and so in this network, as much or more 

 than in the nucleolus, the first division changes are observable. 

 These changes consist in the chromatin of the network becom- 

 ing increasingly prominent and massed. Since some of the 

 threads are clearlv continuous "wath the nucleolus or its mem- 

 brane, and since the nucleolus becomes gradually more and 

 more pale and emptied of its refractive contents as division 

 proceeds, it seems that much of the thread skein or wreath 

 material is derived from the nucleolus. After the aggregating 

 chromatin wreath or so called "spireme- thread" has assumed 

 a definite size, it becomes transversely divided into a number 

 of chromosomes that in number vary for different, and at 

 times even closely related, genera. From the studies of Stras- 

 burger, Flemming, and many more recent workers, we now 

 recognize that each of these chromosomes is made up of def- 

 initely arranged granules or chromospheres, arranged along 

 and embedded in the chromosome substance. These undergo 

 division, some time before division lengthwise of each chro- 

 mosome in which they are embedded, so that the latter becomes 

 dotted lengthwise with pairs of minute chromospheres. 



Soon after the chromosomes have been established, two 

 centers of energy-flow usually appear within or immediately 

 outside the nucleus. These start radiation figures as in Spir- 

 ogyra and so the radiation centers were regarded as important 

 and distinct morphological entities by some, that for a time 

 were called centrosomes. They indicate areas of stereoen- 

 ergizing activity round which the chromatin can gradually 

 redistribute itself in formation of the daughter nuclei. But 

 as yet we are ignorant as to the exact physico-chemical changes 

 that start their formation. The chromosomes next aggregate 

 more or less toward the nuclear center, as if attracted mutually 

 by energizing currents, and they then collectively form what 

 has been called the nuclear or equatorial i)late. 



Each chromosome next splits lengthwise, much as did the 

 minute chromosomes of Spirogyra that made up in it the nucle- 

 olar substance. Then the daugliter chromosomes seem to be 

 driven apart till they reach a position where balance between 

 them and the radiating threads, as well as the surrounding 



