4 INVERTEBEATA chap, i 



changes which the nucleus undergoes previous to, and during, these 

 maturation divisions, have been studied with great minuteness 

 by a large number of observers, and on many important points a 

 general agreement has been arrived at. An excellent summary of 

 the present stage of our knowledge has been given by Agar (1911), 

 who has worked out the development of the male cells in the Dipnoan 

 fish Lepidosiren. 



As maturation approaches, the chromatin granules in the nuclei 

 of the spermatocytes of this species become aggregated into long 

 ribbons, which are the chromosomes. This stage is termed leptonema, 

 and the chromosomes in this stage are termed leptotene threads ; they 

 appear in double the number that are found in the ripe ovum when 

 ready to receive the spermatozoon. This double number is known as 

 2 X. There is a large conspicuous nucleolus present as in the unripe 

 egg (Fig. 1, A, n). The leptotene threads continue to shorten and 

 become thicker and thus they pass into the stage of zygonema, in 

 which the threads become opposed to one another in pairs and are 

 termed zygotene threads (Fig. 1, B). Then the members of each 

 pair fuse with one another. Thus the stage of pachynema is attained 

 in which there are only x shorter thicker pachytene threads or chro- 

 mosomes, arranged in the form of U's in a "bouquet" at one side of 

 the nucleus. 



Here we arrive at a fundamental divergence of opinion between 

 two groups of workers. The changes which we describe cannot, of 

 course, be observed in the living nucleus, but must be inferred from 

 the comparison with one another of fixed and stained nuclei. Some 

 workers maintain that nuclei with x pachytene threads represent the 

 first stage in maturation, and that the stage with 2 x zygotene threads 

 represents an attempt at longitudinal division of these chromosomes, 

 which is, however, abortive. Agar, however, points out that in the 

 material which he studied the leptotene threads cross each other at 

 all angles, and when the process of amalgamation in pairs or syndesis 

 begins, at first only one end of each member of a pair of zygotene 

 threads is parallel with its fellow, the other end passes into an 

 irregular tangle. He, therefore, in common with a great number of 

 workers, interprets the appearances seen in the beginning of the 

 pachytene stage as the beginning of a side-by -side fusion of 

 originally separate chromosomes — i.e. as parasyndesis. This stage 

 is followed by one in which the two elements forming the pachytene 

 chromosome separate in the middle, and the chromosome is trans- 

 formed into an elongated ring which is twisted on itself (Fig. 1, C). 

 This is the stage of strepsinema, the rings being termed strepsitene 

 threads. The two sides of each ring separate from one another, first 

 at one end, so that the ring is converted into an elongated V, and 

 then at the other so that the two original constituents of the ring 

 are again entirely separated from one another and the original 

 number of chromosomes, 2 x, is restored. 



During the strepsinema stage a process called synizesis begins. 



