22 PHYSIOLOGY. 



cumference. By the contraction of the spindle fibers the daughter- 

 chromosomes (the result of the original chromosomes being divided 

 longitudinally into two separate halves by means of fission) are 

 divided into two equal groups, which are moved toward the points, 

 or poles, of the spindle, but never reach it absolutely. Between 

 these groups fine "connecting fibrils" stretch. This figure is called 

 the double star, or diaster. The star shape is formed by the angles 

 of the chromosomes being arranged next to the centrosomes, with 

 their free ends extending out radially. 



There now follows a retransforming of the daughter-chromo- 

 somes, arranged in the form of a star, into a genuine resting nucleus. 

 The angles begin to disappear, the threads draw more closely to one 

 another, becoming more bent and roughened at the same time that 

 little processes appear on their surfaces. A very delicate nuclear 

 membrane develops and surrounds the group of threads. The radi- 

 ating fibers of protoplasm around the centrosomes become more and 

 more indistinct until they finally disappear. The same thing occurs 

 with the "connecting fibrils/' 



When the two daughter-stars are separated as far as possible 

 there appears on the surface of the cell-body a fissure, cutting into 

 the protoplasm in the line of the equatorial plate, until the cell is 

 completely divided into two parts, each containing a nucleus. 



The duration of this process has been seen in man to be half an 

 hour, while in the larvae of the salamander it has been known to take 

 as long as five hours. 



The whole process of mitosis may be divided into five stages : 



1. Prophase (skein stage). 



2. Mother-star stage (monaster). 



3. Metaphase-Metakinesis (diaster). 



4. Anaphase (daughter skeins). 



5. Telophase (daughter nuclei). 



3. Endogenous Nuclear Multiplication. 



A third rare mode of nuclear multiplication, to which is given 

 the above-named title, was discovered in the thalassicola. 



The thalassicola, which is the largest in size of the radiolarians 

 and the diameter of whose central capsule is nearly equal to that of 

 the frog's egg, has, during the major portion of its life, one single,' 

 highly differentiated, giant nucleus, called the internal vesicle. This 

 nucleus, or internal vesicle, usually attains to 1 / 50 inch in diameter, 



