46 CYTOLOGY 



CHAP. 



slit within each bivalent pair in the zygotene nucleus. Such cases as 

 Tomopteris, where there is no synizesis to obscure any stage in the process, 

 seem to be decisive. This fact alone appears to dispose definitety of the 

 theory of telosyndesis. 



2. A considerable degree of similarity between the duplicity of the 

 prophase chromosomes of many somatic mitoses and that of the zygotene 

 pairs must be admitted. The threads which come together in the zygo- 

 tene stage are, however, more distinct than those of somatic prophases. 

 In the leptotene nucleus, or earlier, they often show little or no sign of 

 paired arrangement, in this respect differing greatly from the duplex 

 chromosomes of somatic prophases, where the two longitudinal portions 

 of each chromosome are probably always closely approximated to one 

 •another. Moreover, we do not find in somatic prophase that regular 

 fusion of the members of the pairs spreading away from their polar ends 

 which is such a characteristic feature of the zygotene nuclei in those 

 organisms which exhibit the bouquet orientation in this stage (e.g., 

 Tomopteris, Lepidosiren). The fact, therefore, that the theory of 

 parasyndesis has to interpret the frequent duphcity of somatic prophase 

 chromosomes as caused by fission, and the superficially somewhat similar 

 duplicity of the zygotene nucleus as fusion, is no serious drawback to 

 that hypothesis — especially when it is remembered that, as will be 

 evident from the next paragraph, such a differentiation must in any 

 case be made between the double prophase chromosomes of Ctdex and 

 those of most other organisms. 



3. There is commonly observed, even in somatic mitoses, a tendency 

 for homologous chromosomes to lie side by side, and this tendency can 

 be traced through all intermediate degrees up to its climax in Ctdex, 

 where even in somatic, spermatogonial and oogonial mitoses, homologous 

 chromosomes may be indistinguishably fused together, especially in 

 prophase and anaphase. It is instructive to compare Figs. 14, I, 17, I, 

 19, B, E, with Fig. 56, C, D, E, H, remembering that in the latter there is 

 no questioning the fact that the longitudinal components of the double 

 chromosomes are each an entire chromosome, the pairs being formed by 

 approximation of these, and not by fission of a single original one. Para- 

 syndesis is therefore but the climax of a widespread tendency of homolo- 

 gous chromosomes to apply themselves side by side. 



4. As stated in Chapter I., a tendency for chromosomes to adhere by 

 their ends is sometimes observed in somatic mitoses. This may be taken 

 as favourable to the view that syndesis is effected in the same way, i.e. 

 by telosyndesis. 



5. There is no direct evidence that the pachytene loops consist of 

 two chromosomes united end to end. The fact that a break is often 

 observed in the loop, dividing it into two portions connected by an 



