GENERAL EMBRYOLOGY 1599 



Spireme. — ^The chromatin and linin of the nuclear reticulum and 

 nuclear membrane become transformed into a coiled thread, called 

 the spireme or skein. 



Chromosomes. — ^The spireme is broken up transversely into an 

 even number of segments, called chromosomes, the number of these 

 being constant and characteristic of the species of animal. These 

 chromosomes usually assume the form of short rods, which resemble 

 a V. 



Spindle. — ^Whilst the chromosome-stage is in progress, important 

 changes take place in the stellate centrosome or aster. It divides 

 into two segments, each division taking up a central or attraction- 

 particle, and being furnished with radiating fibres. In this manner 

 two centrosomes or asters are formed. Certain of the radiating 

 fibres extend from one centrosome to the other in a fusiform manner, 

 and these connecting fibres, called the spindle-fibres, constitute the 

 spindle, which has a centrosome or aster at either pole. As the 

 nucleus becomes elongated transversely, the two centrosomes take 

 up positions one at either pole of the somewhat elliptical nucleus, 

 the spindle-fibres becoming gradually elongated. Up to this point 

 the spindle, with an astral centrosome at either pole, is external 

 to the nuclear membrane, but when this membrane disappears the 

 spindle becomes intranuclear, and the spindle-fibres extend from 

 one pole of the nucleus to the other, where they are connected with 

 the two astral centrosomes respectively. 



The foregoing phenomena conclude the anaphase or preparatory 

 stage. 



Metaphase. — ^After the disappearance of the nuclear membrane, 

 the chromosomes are brought into direct contact with the spindle, 

 and he at first scattered between the spindle-fibres. Very soon, 

 however, they congregate at the equatorial plane of the spindle, 

 which corresponds to its widest part. Here they are arranged in a 

 stellate manner, which constitutes the aster according to some 

 authorities. Each chromosome now splits longitudinally into two 

 equal parts, called daughter-chromosomes, the original nimiber of 

 parent -chromosomes being thereby doubled. The formation of 

 daughter-chromosomes constitutes the metaphase or chief stage. 



Kataphase. — ^The daughter-chromosomes at first form two rows 

 at the equatorial plane of the spindle, lying close to each other. 

 They soon, however, separate, those of each row travelling along 

 (tnetakinesis) the corresponding spindle-fibres to either pole of the 

 spindle, where they enter the centrosome or aster. These pheno- 

 mena conclude the kataphase, or leading ' down ' stage. 



Telophase. — The daughter -chromosomes within each aster now 

 imite end to end, and form a spireme, round which a new nuclear 

 membrane is formed. The spireme gradually assumes the form 

 of a chromatic reticulum, characteristic of a normal nucleus, and 

 karyoplasm is formed within the meshes of the reticulum. Two 

 daughter-nuclei are thus constructed, one in either centrosome, 

 each of which contains one-half of the parent -chromosomes belong- 



