28 TEXT-BOOK OF PHYSIOLOGY. 



which move apart to positions at the opposite ends of the long axis of 

 the nucleus. At the same time delicate achromatin fibers make their 

 appearance, arranged in the form of a double cone, the apices of which 

 correspond in position to the centrosomes. This is known as the 

 nuclear spindle. During the prophase the nuclear membrane and the 

 nucleoli disappear. 



2. The Metaphase. The two centrosomes are at opposite ends of the long 



axis of the nucleus, each surrounded by an attraction sphere, now 

 called the polar radiation. The chromosomes become yet shorter and 

 thicker, and move toward the equator of the nucleus, where they lie 

 with their closed ends toward the axis, presenting the appearance, 

 when seen from the poles, of a star the so-called mother star, or mon- 

 aster. While moving toward the equator of the nucleus, and often 

 earlier, each chromosome undergoes longitudinal cleavage, the sister 

 loops remaining together for a time. Upon the completion of the 

 monaster, one loop of each pair passes to each pole of the nucleus, 

 guided, and perhaps drawn by the threads of the nuclear spindle. The 

 separation of the sister segments begins at their apices, and as the open 

 ends are drawn apart they remain connected by delicate achromatin 

 filaments drawn out from the chromosomes. This separation of the 

 daughter chromosomes, and their movement toward the daughter 

 centrosomes, is called metakinesis. As they approach their destination, 

 we have the appearance of two stars in the nucleus the daughter stars, 

 or diasters. 



3. Anaphase. The daughter stars undergo, in reverse order, much the 



same changes that the mother star passed through. The chromo- 

 somes become much convoluted, and perhaps united to one another, 

 the lateral twigs appear, and the chromatin resumes the appearance 

 of the resting nucleus. The nuclear spindle, with most of the polar 

 radiation, disappears, and the nucleoli and the nuclear membrane 

 reappear, thus forming two complete daughter nuclei. Meanwhile 

 the protoplasm becomes constricted midway between the young nuclei. 

 This constriction gradually deepens until the original cell is divided, 

 with the formation of two complete cells. 



Physiologic Properties of Bioplasm. All living bioplasm possesses 

 properties which serve to distinguish and characterize it viz., irritability, 

 conductivity, and motility. 



Irritability, or the power of reacting in a definite manner to some form 

 of external excitation, whether mechanic, chemic, or electric, is a funda- 

 mental property of all living bioplasm. The character and extent of the 

 reaction will vary, and will depend both on the nature of the bioplasm and 

 the character and strength of the stimulus. If the bioplasm be muscle, 

 the response will be a contraction; if it be gland, the response will be a 

 secretion; if it be nerve, the response will be a sensation or some other form 

 of nerve activity. 



Conductivity, or the power of transmitting molecular disturbances arising 

 at one point to all portions of the irritable material, is also a characteristic 

 feature of all bioplasm. This power, however, is best developed in that form 

 of bioplasm found in nerves, which serves to transmit, with extreme rapidity, 



