20 



THE NUCLEUS IN MATURATION 



separated without further splitting (fig. 30, c). In very many cases there is 

 no reconstruction of the nucleus between the heterotype and the homotype 

 division, but in others a partial reconstruction takes place, without however a 

 loss of the identity of the halves of the double anaphase figures of the heterotype. 

 The case of true tetrads involves certain difficulties, but the explanation of 

 their occurrence seems to be that the second, or anaphase cleavage, which is fore- 

 shadowed in the prophase in many forms, has been completed before the formation 

 of the first spindle. There has been much diversity of opinion as to the character 

 of the second cleavage, whether it is a longitudinal splitting or transverse breaking 

 of the spireme-thread. If it be a longitudinal splitting of the elements of the 

 ' synaptic gemini,' the case falls in with the scheme adopted. 



Difficult as the stages are to follow during 

 maturation of the sperm-cells, they are still further 

 complicated during the maturation of the egg, by 

 the fact that an immensely long period of repose 

 occurs between the early stages of the hetero- 

 typical prophase and the actual division of the 

 nucleus, during which the nucleus assumes the 

 vesicular form characteristic of the ripening 

 oocyte (fig. 31). As the oocytes are all formed 

 before birth, the beginning of the prophase must, 

 in the human subject, be separated by many 

 years from the actual division. The manner in 

 which the chromosomes are re-formed out of the 

 germinal vesicle when maturation sets in, is by 

 no means cleared up. The chroma tin-threads are, 

 according to competent observers, completely lost 

 sight of in the vesicle, and the chromatin gathered 

 into nucleoli, which again give rise to chromo- 

 somes. This is a difficulty with which the theory 

 of persistence of the chromosomes has to contend ; 

 but in any case double-rod figures appear, which 

 pass into the metaphase (fig. 32) exactly as in 

 spermatogenesis. In maturation of the egg in 

 Echinus the double-rod figures (pseudotetrads) are 

 attached to the spindle by their extremities. The 

 figures, though minute,, are simple, and are here 

 reproduced as an example of the process (fig. 33). l 



FlG. 29. DlAGBAM BEPBESENTING 

 THE BEHAVIOUR OF THE DOUBLE 

 CHBOMOSOMES OF THE HETEBO- 



TYPICAL DIVISION. (After Gregoire.) 

 a, Three double chromosomes, 

 variously arranged ; &, the mode of 

 attachment to the spindle of the three 

 types 1, near the ends; 2, 'opposite 

 the middle points ; 3, at the ends ; 

 c, resulting double anaphase figures 

 1, tailed V's ; 2, simple V's ; 3, straight 



Significance of the nuclear phenomena. It 



is impossible here to deal at length with the history 



of opinion on the interpretation and significance of the phenomena described above. In the 

 last edition of this work two theories were briefly alluded to the sex theory of Minot 

 and the hypothesis of Weismann. Minot's tHeory, which applied to the ovum only, postulated 

 that the cells were hermaphrodite, and supposed that in the extrusion of the polar bodies the male 

 element was got rid of from the ovum, to be replaced again in fertilisation. This became 

 untenable when the parallelism of the maturation processes in spermatogenesis and oogenesis 

 was established, and it was proved that all four elements resulting from the division of the 

 spermatocytes became functional spermatozoa, while only one of the products of the division of 



1 The following works may be consulted on this subject : Ed. B. Wilson, The Cell in Inheritance 

 and Heredity, 1900 ; Wmiwarter, Arch, de Biol. 1900 ; Korschelt and Heider, Lehrbuch d. vergleich. 

 Entwickelungsgeschichte d. wirbellosen Tiere, Lieferung ii. 1903. Gregoire. La Cellule, xxii. 1905, gives 

 a useful summary and bibliography to date of publication. Since then, among other papers, have 

 appeared Farmer and Moore, Quart. Jour. Micro. Sc. xlviii. ; J. E. Lane-Clay pon, Proc. K. S. 1905; 

 Janssens, La Cellule, xxii. ; A. and K. E. Schreiner, Arch, de Biol. xxii., and Anat. Anzeiger, xxix. ; 

 Van der'stricht (bat), Compt. rend, de 1'Assoc. des Anat., 8 Reunion, Bordeaux, 1906 ; Moore and 

 Walker, The Meiotic Process in Mammalia, Rep. Cancer Research Lab., Univ. Liverpool, 1906. 



