136 GENERAL EMBRYOLOGY 



series of developmental processes, and it is at the same time the 

 starting point of the life cycle of a new organism of another 

 generation. 



~~In a few forms (e.g., some Insects, Lepidosiren, Fig. 72), the 

 double nature of these bivalent chromosomes is distinctly visible 

 and is indicated by a split through the long axis of the chromo- 

 some, showing that the pair of univalent elements have fused 

 side by side, a condition known as parasynapsis. In most 

 cases observed (other Insects, Amphibia) the fusion is end to 

 end, a condition known as telos^na^sis (Wilson's terms). In 

 many instances, however, the fusion seems to have occurred 

 between the granules composing the chromosomes, so that in 

 the bivalent body there is no visible indication of the duplex 

 nature at this time; this is then only to be inferred from the 

 fact of numerical reduction. It is very important to notice 

 that the time relations between synizesis and synapsis may 

 sometimes be just the reverse of that described above, and the 

 synapsis stage may occur first, so that the numerical reduction 

 of the chromosomes occurs at the close of the last spermatogo- 

 nial division (some plants, Strasburger, Overton). 



following the period of synapsis the nucleus and cell may 

 proceed at once to divide, or there may ensue another resting 

 period, during which the chromosomes again become indistinct. 

 In either case, when the new mitotic figure forms, always after 

 an unusually long prophase which is characteristic of this divi- 

 sion, the reduced (haploid) and bivalent chromosomes often 

 show an unusual condition, in that they may prepare at once, 

 not for a single ensuing division, but for two divisions which are 

 to follow immediately, without an intervening resting period. 



From this stage onward in the history of the sperm and egg 

 nuclei, two general types of chromosome behavior are some- 

 times distinguished, although they are connected by transi- 

 tional conditions and so are regarded as modifications of a 

 single process. As one extreme condition we find a form of 

 chromosome behavior called tetrad formation, which we may 

 describe, not because it is a typical method of chromosome 

 reduction, but because the facts of reduction come out more 



