DIV. I 



MORPHOLOGY 205 



formed ( 95 ). Such eggs, which already have the double number of chromosomes 

 usually only attained on fertilisation, proceed to develop without fertilisation. 

 This is the case for the unfertilised egg-cells of the Sperm atophyta, Marsiliaceae, 

 and Chara, mentioned on p. 193, while in other Algae the haploid egg-cell 

 develops parthenogenetically into a new plant. When diploid sexual cells proceed 

 to develop without fertilisation, it is usual to speak of apogamy (cf. p. 202) and not 

 of parthenogenesis. 



The reduction division in contrast to the typical division is termed HETERO- 

 TYPIC, and is also spoken of as MEIOSIS. It is characteristic of this, that 

 in the prophase the nuclear contents become for a period contracted together at 

 one side, at least in fixed preparations (SYNAPSIS, Fig. 233, 2, 3). It is 

 further characteristic of the succeeding stages that the paternal and maternal 

 chromosomes become associated or united in pairs or GEMINI. The number of 

 these GEMINI is half as great as the number of chromosomes in [the tissue cells 

 of the same plant, since two chromosomes are represented by each segment. The 

 paired chromosomes become shorter and thicker and are distributed around 

 the periphery of the nucleus ; this is the condition that has been termed 

 DIAKINESIS (5, 6). At this stage kinoplasmic filaments are becoming applied to 

 the nuclear membrane (6) ; the latter disappears, and the nuclear spindle, which 

 is at first multipolar (7) but ultimately becomes bipolar (8), originates from the 

 kinoplasmic fibres. The paired chromosomes become attached to the fibres of the 

 spindle and arranged in an equatorial nuclear plate (8). Shortly afterwards the 

 separation of the chromosomes, until now united in pairs, takes place (9). IN 



THIS PROCESS, IN WHICH THE ESSENTIAL OF THE REDUCTION DIVISION IS EFFECTED, 

 IT IS NOT LONGITUDINAL HALVES OF CHROMOSOMES BUT ENTIRE CHROMOSOMES 



WHICH SEPARATE FROM ONE ANOTHER. The result of this is that each daughter 

 nucleus receives only half as many chromosomes as were found in the tissue cells 

 of the same plant, and that these chromosomes may be male or female. Since 

 chromosomes of corresponding lengths are always associated in the gemini, one 

 being derived from the male and the other from the female parent, and these 

 chromosomes separate from one another in the reduction division, each haploid 

 daughter nucleus must inherit some chromosomes from the father, and others 

 from the mother. Which chromosomes come from the one or other parent appears 

 to be determined by chance. The formation of the daughter nuclei is completed 

 (10) 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, which in all essentials follows the typical course (11, 12). 

 Thus two rapidly-succeeding nuclear divisions are characteristic of most cases of 

 reduction. In the homo ty pic division longitudinal halves of chromosomes separate 

 as in the typical division. A difference from the latter is that the chromosomes 

 are not split longitudinally in the prophase of the homotypic division itself, but, 

 as it seems, were already split in the prophase of the preceding reduction division 

 without the halves thus formed separating. 



The fundamental difference between the typical and somatic nuclear division 

 and the reduction division may be made clearer by means of a diagram. Fig. 234 

 A represents a somatic division with longitudinal splitting of the Chromosomes. 

 In A a six longitudinally split chromosomes, distinguished by the different 

 shading, are shown arranged to form the nuclear plate. The two middle ones are 

 seen from the end, the others from the side. In A b the separated halves of these 

 chromosomes are shown on their way to the poles of the spindle in order to form 

 the daughter nuclei. In Fig. 234 B the reduction division is diagrammatically 

 represented. The six chromosomes of Fig. 91 A are shown in B a similarly 



