GENERAL INTRODUCTION. 7 



Vejdovsky,' that the central mass of the aster — i.e., the centrosome — does not disappear at the close of cell-division, but 

 persists as a permanent cell-organ lying beside the nucleus, as already described fText-fig. V. D). This body forms the 

 starting-point of the amphiaster for the succeeding cell-division, dividing into two halves, each of which becomes surrounded 

 by astral rays, while between them is developed a spindle, formed in part, at least, out of the linin network of the nucleus. 

 The chromatic substance of the nucleus is meanwhile transformed into chromosomes which arrange themselves about 

 the equator of the spindle. This discovery of Van Beneden's, originally made in the case of Ascaris, has now been 

 confirmed in a large number of other animals and in plants, and the centrosome has come to be generally regarded as a 

 permanent cell-organ, which presides over the formation of the amphiaster by which the operation of cell-division is effected 

 and the nuclear material is distributed to the daughter-cells. 



What now is the origin of the centrosomes in the fertilized egg.'' Is their substance, like that of the cleavage-nucleus, 

 derived ecjually from the two sexes.'' If the centrosome is like the nucleus, a permanent and necessary organ of the 

 cell, then it must be present in each of the germ-cells, as in other cells. On this purely a priori ground Rabl asserted, 

 in the year iSSg,- that the fertilization of the ovum must involve a union not only of nuclei, but also of centrosomes. 

 This prediction was apparently realized in the year 1S91, when Fol described in the sea-urchin precisely such a conjuga- 

 tion of centrosomes, as the theory demanded.^ A similar result was soon afterwards reached by Guignard in the case of 

 the lily,^ and still later by Conklin, in the marine gastropod, Crepidulal' 



These conclusions regarding the origin of the centrosomes were, however, opposed to those of most earlier observers, 

 all of whom agreed that they were derived solely from the sperm-cell. In Toxopneustes, the latter appears to be certainly 

 the case.'' The amjDhiaster is in this case developed under the influence of a body contained in the spermatozoon, and 

 located in that portion of the sperm-cell known as the middle-piece, which lies behind the nucleus between it and the 

 tail' (Text-fig. II. A ; see also p. 13). This body is a centrosome, and about it as a centre, after the spermatozoon enters 

 the egg, is developed a single aster, known as the spenn-aster, which accompanies the sperm-nucleus in its jarogress 

 towards the egg-nucleus (Text-figs. IX., X.), and finally divides into two daughter-asters which place themselves at 

 opposite poles of the cleavage-nucleus (Text-figs. XII., XIII.). These daughter-asters become those of the first 

 cleavage-amphiaster, while between them is developed the spindle from the achromatic substance of the nucleus itself. 

 A precisely similar result has been reached in the case of several other animals; in the Axolotl, by Fick-; in Ascaris, 

 by Boveri-'; in i\\c Annelid C/ia-iopieriis, by Mead'"; in the starfish, by Mathews ''; in Rhynchclmis, by Vejdovsky ^^; 

 and in several other cases. On the other hand, Wheeler has found in Myzostomum that the centrosome is derived from 

 the egg-cell alone and not from the sperm-cell. ^^ 



Fertilization and Inheritance. — These facts seem to demonstrate conclusively that the centrosome cannot play any 

 part in hereditary transmission, since it maybe derived from one sex only. It is rather to be regarded in accordance with 

 Van Beneden and Boveri as a dynamic element, by means of which is organized the machinery of cell-division. In such 

 cases as Toxopneustes, the original centrosome of the egg-cell degenerates and disappears. The egg is, therefore, not 

 capable of cell-division until a new centrosome appears within it. The egg is unable to develop its own centrosome ; 

 this organ must therefore be derived from an outside source, namely, the spermatozoon. Fertilization, accordingly, consists 



^Vejdovsky. See Anatomischer Anzeiger, VI., 1891. p. 370. trosome of the testis-cell ('-spermatocyte"), from which the spermatozoon 



- Anat. Anzeiger, IV., 1891. p. 29. arises (Hermann, in the Salamander; Calkins, in Liitjibnais). 



3 Anat. Anzeiger, VI., 1891 ; also Arch. Sci. Phys. et Nat., XXV., 1891. 8 Zgit. Wiss. Zool., LVI., 4, 1893. 



*Ann. d. Sci. Nat., Bot., 14, 1891. » Zellstudien, II., 1888. 



5 Wood's Holl Biological Lectures, II., 1893. «. jg^rn. Morph., X., I, 1895. 



«See Wilson and Mathews, Journ. Morph., X., 1895. p. 319. " Journ. Morph., X., i. 1895. 



'The middle-piece has been traced back in the spermatogenesis to the cen- ^- Entwick. Untersuch., I., 1888. 



1' Journ. Morph., X., i. 1895. 



