II MEIOSIS IN TOMOPTERIS 



33 



An early stage in the preparation for the first meiotic division is 

 shown in Fig 14, D, where it can be seen that the chromatin is condensing 

 into fine threads, and also that (i) this condensation is most marked at 

 one pole of the nucleus (shown throughout the figure as the upper pole), 

 and (2) at this pole the chromatin threads converge in pairs. Antici- 

 pating, we may say, that each chromatin thread (of which there are 

 eighteen) is a chromosome, and that the pairing is not haphazard, but 

 that each pair consists of two homologous chromosomes in the sense 

 described on pp. 13 and 125. 



In favourable preparations it can be determined that the centrosome 

 is embedded in the cytoplasm just outside that pole of the nucleus to 

 which the chromosomes converge. 



Another important point to notice is that the condensed chromosomes 

 at the pole of the nucleus are not smooth, but resemble strings of 

 beads. These beads are the chromomeres, and will be further discussed 

 in Chapter V. This stage (Fig. 14, D), where the chromosomes are still 

 very fine, is known as the leptotene stage. 



In the later stage, shown in Fig. 14, E, the condensation has spread 

 away from the pole along a further length of the chromosomes, and now 

 the homologous chromosomes which were paired in Fig. 14, D, are 

 beginning to approximate themselves still more closely, till they come into 

 actual contact. Like the preliminary condensation, this process begins 

 at the polar end of the chromosomes and spreads away from this point. 

 This coming together of pairs of chromosomes, which is of fundamental 

 importance, is often known as the conjugation of the chromosomes from its 

 resemblance to the conjugation of certain Protozoa, especially Infusoria 

 such as Paramecium. It is also known as syndesis, while the nucleus is 

 said to be in the zygotene stage. 



Fig. 14, F, shows a more advanced stage of syndesis, and illustrates 

 also the fact that the process is not necessarily synchronous in all the 

 chromosomes. In the nucleus shown in the figure, three pairs of chromo- 

 somes are still in the leptotene stage. 



In Fig. 14, G, syndesis is complete, and now instead of the eighteen 

 thin chromosomes of the leptotene stage we have nine thick chromosomes 

 formed by their fusion in pairs. Hence this stage is called the pachytene 

 stage. The chromosomes are now seen to be horse-shoe shaped, with their 

 ends directed towards the nuclear pole. A characteristic appearance 

 is thus produced in the pachytene nuclei of Tomopteris and of many other 

 forms which exhibit a similar polarization. This has earned for this 

 stage the further term of bouquet stage. In many species, however, such 

 a polar orientation is absent. 



The chromosomes have by this time condensed and contracted 

 sufficiently to make it possible to count them, and it is found that there 



D 



