86 BOTANY part i 



originates from the kinoplasmic fibres. The paired chromosomes 

 become attached to the fibres of the siiindle and arranged in an 

 equatorial nuclear plate (10). Shortly afterwards the separation of 

 the chromosomes, until now united in pairs, takes place (11). In this 

 process, in which the essential of the reduction division is effected, 

 i t is not long itudinal halves of c hromosomes but entire chromosomes 

 whic h, sepnrntn from one anoth er. The re s u lt of this is that eac h_ 

 daughter nurlfi vs receives only half as many chromos omes as avp.tp 

 fo und in the tissue cells of the same plan t. During their passage 

 towards the poles of the spindle a longitudinal split can be detected 

 in each chromosome. This split Avas indeed alreadj' 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 foiniiation 

 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. In this no new longitudinal splitting of the chromo- 

 somes 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 Avith 

 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 show^s 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 cpiickly. The heterotype and homotype nuclear divisions, which 

 may together be termed the meiotic (meiosis) or allotypic division, 

 may be contrasted with the ordinary or typical nuclear division ; this 

 latter is also known as somatic division. At a particular stage of 

 development corresponding phenomena to those of the meiotic division 

 are met Avith in animals as Avell as in plants. 



The smallest reduced number of gemini known for tlie nuclei of the sexual 

 generation in the more highly organised jilants is three (in the Composite Crcjns 

 virens) ; this is the half of the smallest number found in the nuclei of the corre- 

 sponding asexual generation. 



For certain of the lower Cryptogams, e.rj. the Sea-weed Fucus, a division into 

 two of the centrosonie-like body (p. 53) in relation to the nucleus is recorded at 

 the commencement of karyokinesis. The halves separate from one another (Fig. 

 89 c) and ultimately come to occupy the poles of the nuclear spindle. Round the 

 cytoplasm investing these centrosome-like bodies, kinoplasmic radiations form 

 (Fig. 89 kp). When the two bodies have reached the poles of the nucleus, the 

 nuclear membrane disappears at these points and si)indle fibres appear in the 

 nuclear cavity and attach themselves to the chromosomes. The complete nuclear 



