II MEIOSIS IN TOMOPTERIS 



35 



daughter chromosomes, which will become operative in the succeeding 

 mitosis. In the present case, that mitosis is the second division of the 

 meiotic phase which follows immediately after the first without the 

 intervention of a resting stage. It is depicted in Fig. 14, N, 0, P, and does 

 not differ essentially from a somatic mitosis except in having only half 

 the normal number of chromosomes. The last figure illustrates a not 

 uncommon minor feature of spermatogenesis, namely, that the meiotic 

 mitoses are not immediately followed by complete cell division, so that 

 for a time the four young spermatids are united in a four-lobed cell. 



At this point we may leave the description of spermatogenesis in 

 Tomopteris, all the important nuclear phenomena being by now concluded. 

 The development of a spermatid into a spermatozoon is described in the 

 next chapter. 



The fundamental fact in meiosis is the segregation, into separate nuclei, 

 of the members of each pair of homologous chromosomes. This is brought 

 about in the meiotic mitosis by the previous pairing of the homologues into 

 double or bivalent chromosomes, which take up a position on the spindle such 

 that the constituent chromosomes of each bivalent occupy the position taken 

 in an ordinary mitosis by the daughter halves of each single chromosome. 

 Thus the meiotic anaphase separates whole {homologous) chromosomes, 

 instead of the daughter halves of single chromosomes, and therefore the 

 daughter nuclei have only half the number of chromosomes present in the 

 previous cell generations. In other words, the pre-meiotic nuclei have 2n 

 chromosomes, the primary spermatocyte has n double or bivalent chromosomes, 

 and the post-meiotic nuclei have n chromosomes. 



The course of meiosis just described is schematized in Fig. 15, which 

 is a diagrammatic representation of its course in a species with four 

 chromosomes. Fig. 15, A, shows a pre-meiotic (spermatogonial) prophase. 

 The remaining figures illustrate the fundamentally important fact that 

 it is homologous chromosomes which pair together in syndesis, to break 

 apart again in the diplotene stage, and finally separate in the first meiotic 

 division. The proof of the statement that syndesis takes place between 

 homologous chromosomes is found in the large number of species in 

 which the chromosomes are of different lengths and even different shapes, 

 so that homologous chromosomes can be identified by their relative 

 sizes, just as they can be recognized in the diagram by their shading. 

 Species with chromosomes of varying lengths are very numerous and 

 will be met with frequently in the accounts in this book (see especially 

 p. 125). One of them, Lepidosiren, will be described immediately. 



It is clear that, in the case illustrated, meiosis will result in gametes 

 having two instead of four chromosomes, and, moreover, that these 

 two will consist of one member of each of the two pairs present in the 



