1902.] NICHOLS— SPER:\rATOGENESIS ONISCUS ASELLUS LINN. 93 



6. Summary. 



The main results of this study may now be briefly summarized as 

 follows : 



(i) The spermatogonia chromosomes are joined together in 

 pairs in the synapsis to form sixteen bivalent chromosomes. They 

 may be joined {a) in an approximately straight line, {I)) to form a 

 more or less narrow V, or {c) into a more or less complete ring 

 (Figs. 26, 27). 



(2) A longitudinal splitting of the chromatin threads takes place 

 at this stage (Figs. 25^, by c). 



(3) The distinctness maintained by the chromosomes up to the 

 formation of the nuclear network of the resting spermatocyte, and 

 the manner of origin of the spermatocytic chromosomes from it, 

 lends support to the theory of their individuality in the resting 

 nucleus (Figs. 28 and 32). 



(4) In the structure and mode of origin of the bivalent sper- 

 matocytic chromosomes two main types may be distinguished : 

 (a) The component chromosomes lie end to end, or {b) they lie 

 side by side (Figs, d'^a, b, c). 



(5) Inasmuch as univalent chromosomes are separated, the first 

 maturation division is reductional (Figs. 48-59). 



(6) Sphere substance (idiozome) is not observable, except for a 

 short time during the prophases of the first spermatocyte (Figs. 

 39 and 43). 



(7) The nucleolus of the spermatogonia disappears shortly after 

 dissolution of the nuclear membrane, while that of the spermato- 

 cytes, first discovered in the synapsis, persists throughout the 

 divisions (Figs. 8-10, 47, 26, 29, 2iZ, 47, 48, 51^ 52, 55^ S^, 60a, 

 61, 65-67, 69). 



(8) The spermatids become associated in groups to form colonies 

 of nuclei lying in a common plasma (Figs. 73-75). 



(9) Within the latter arise bundles of fibres of great length, 

 whose connection with the nuclei, if actual, is very slight and 

 occurs very late, as well as single fibres of greater delicacy which 

 are continuous with the nuclei (Figs. 76-83). 



(10) The mature sperm colony consists of a variable number of 

 filamentous nuclei contained, together with the bundle of cyto- 

 plasmic fibres, in a tenuous sheath which is flagellate at its anterior 

 extremity (Figs. 84-86). 



