APPROACH AND CONJUGATION OF THE NUCLEI, ETC. 21 



Plate VI. Phototype 22. 

 The " Quadrillcr Double-fertilized Egg during the Pause. 



This specimen is the result of a double fertilization such as is shown in No. 12. The two sperm-nuclei have 

 completelv fused to form a single large cleavage-nucleus. Each siaerm-aster has divided into two, thus giving rise 

 to four asters arranged in a symmetrical tetraster. The egg subsequently divides into four instead of two 

 (see p. 16). 



Fol, in 1 89 1, described a "Quadrille of Centres" in the sea-urchin egg {Strojigylocentrotus lividus) in which 

 two centrosomes derived from the spermatozoon were supposed to conjugate with two corresponding egg-centro- 

 somes. No such process can be seen in Toxopnaistcs, and it is possible that Fol may have been deceived by such 

 specimens as that here shown. 



Plate VI. Phototype 23. 



The "Pause." Showing especially the Cleavage-nucleus. 



The nucleus is elongated preparatory to division. The chromatic reticulum is here very clearly brought out. At 

 the upper margin is a rounded " nucleolus." The asters are now at a minimum, the rays scarcely apparent. 



(Journ. Morph., VIIL, 1893"), and Reinke would distinguish in the cell primary, secondary, and tertiary centres (Arch. .Mik. .^nat.. XLIV.. II., p. 276, 1894), which 

 may be formed at any point " je nach dem Bediirfniss der Zelle,"' the centrosomes being primary centres and formed by the aggregation of tertiary centres or microsomes. 



All these considerations, and many others which cannot here be reviewed, indicate, in the author's opinion, that the centriole is essentially an effect and not a 

 cause ; that it is a product of a specific form of metabolic action in the " centrosome " of which it forms a part, rather than the inciting cause of such activity. 



As regards Toxopneustes, the centrospheres show with such beautiful clearness, not only after treatment with sublimate-acetic but also after Flemming's fluid (and in 

 a less degree after pure sublimate), and stain so sharply, that a total destruction of the centriole appears highly improbable. The reticulated centrosphere maybe 

 traced back to the group of one, two, or more minute granules visible in the centre of the aster during the early "pause," and a similar group of granules often appears 

 in the centre of the sperm-aster in cases of polyspermy, when the aster remains long undivided. In the normal undivided sperm-aster they cannot, as a rule, be seen, 

 though occasionally present. These granules I believe to correspond to the centrioles of other forms ; and the evidence indicates that they are formed endogenously in 

 the aster. and by their growth and multiplication give rise to the reticulated substance of the centrosphere (centrosome, in Boveri's sense). Tliis process is obviously 

 nearly related with that described by Vejdovsky in the case of i?/y«c/z^///«'.f (I.e.), though differing from it in the fact that the formation of the daughter-centrioles 

 does not occur until after division of the central mass of the aster as a whole. 



