86 



BOTAXY 



same plant. During their passage towards the poles of the spindle 

 a longitudinal split can be detected in each chromosome. This split 

 was indeed already complete in the prophase before the nuclear plate 

 was formed, but was not followed as in an ordinary division by a 

 separation of the halves. The two halves of each chromosome 

 remain on the other hand in relation to one another and pass to the 

 ? same daughter nucleus. The formation of the daughter nuclei is 

 completed (12) as in an ordinary division, but following promptly on 

 the first reduction division, which is also known as the HETEROTYPE 

 division, comes a second or HOMOTYPE division ( 84 ). In this no new 

 longitudinal splitting of the chromosomes takes place, but the two halves 

 of each chromosome, which existed in the daughter nuclei, become 

 separated from one another, and become the chromosomes of the 

 grand-daughter nuclei. 



The steps of this homotype division agree in other respects with 

 those of an ordinary nuclear division, and will be clear from Fig. 88, 

 13-16. In 13 an early 'stage and in 14 the completed condition of 

 the spindles of the dividing daughter nuclei are seen; 15 shows the 

 division of the nuclear plate, and in 16 the young grand-daughter 

 nuclei are completed. One of the characteristic features of the whole 

 process is that the two divisions succeed one another immediately 

 or very quickly. The heterotype and homotype nuclear divisions, 

 which may together be termed the ALLOTYPIC division, may be con- 

 trasted with the ordinary or typical nuclear division. At a particular 

 stage of development corresponding phenomena to those of the allo- 

 typic division are met with in animals as well as plants. 



The smallest reduced number of chromosomes known for the nuclei of the 



more highly organised plants is four, i.e. the 

 half of the smallest number met with in the 

 tissue cells. 



In those lower Cryptogams, the nuclei of 

 which possess an individualised centriole, the 

 latter undergoes division into two at the be- 

 ginning of karyokinesis. The two halves 

 from one another (Fig. 89 c) and 

 ultimately reach the jioints which will become 

 the poles of the spindle. Round such cen- 

 ~' ^LyT/'-v ; trioles a definite portion of protoplasm form- 



?~\ ing the centrosome is usually marked off, 



and around this kinoplasmic radiations (kp) 

 FIG. MI. A nucleus of a vouii" plant of ,. v IITI. j.i j_- i 



form an astrosphere. When the centnoles 

 the Brown Seaweed, Fitcus serratus, 



preparing to divide. The two centrioles have reached the poles the nuclear membrane 

 (c), which have arisen l.y the division disappears, and spindle fibres appear in the 

 of a sin-le one, have already separated nuclear cavity itself. These clearly proceed 

 from one another;/.-?, radiations of the from the cent rosomes and become atta.-h.-<i 

 nbnllar plasma; s, chromosomes ; _, 1.1.1 



nucleolus (x 1000) to ^ e chromosomes. The complete nuclear 



spindle (Fig. 90) has a centrosome with kino- 

 lasmatic radiations (kp] at each pole, but in other respects agrees with the spindles 



,"> v. j j; - 



* .- ' , -.'>," , ginning 



^.y: '..-.; * pte 



X-^ltfePRSr^? ultimate! 



^rV^ESag - the pole, 



