THE SEGMENTATION OF THE OVUM. 91 



Without attempting a full discussion of the subject I will confine myself to 

 a few remarks suggested by the observations of Flemming, Peremeschko and 

 Klein. The observations of these authors shew that in the course of the 

 division of nuclei in the salamander, newt, etc. the nuclear reticulum under- 

 goes a series of peculiar changes of form, and after the membrane of the 

 nucleus has vanished divides into two masses. The masses form the basis 

 for the new nuclei, and become reconverted into an ordinary nuclear reticu- 

 lum after repeating, in the reverse order, the changes of form undergone 

 by the reticulum previous to its division. 



It is clear without further explanation that the conversion of the 

 nuclear reticulum of the segmentation nucleus into the striae of the spindle 

 is a special case of the same phenomenon as that first described by Flemming 

 in the salamander. There are however some considerable differences. In 

 the first place the fibres in the salamander do not, according to Flemming, 

 unite in the middle line, though they appear to do so in the newt. This clearly 

 cannot be regarded as a fact of great importance ; nor can the existence of 

 the central thickenings of the striae (nuclear plate), constant as it is for the 

 division of the nucleus of the ovum, be considered as constituting a funda- 

 mental difference between the two cases. More important is the fact that 

 the striae in the case of the ovum do not appear, at any rate have not been 

 shewn, to form themselves again into a nuclear network. 



With reference to the last point it is however to be borne in mind (i) that 

 the gradual travelling outwards of the two halves of the nuclear plate is 

 up to a certain point a repetition, in the reverse order, of the mode of 

 formation of the striae of the spindle, since the striae first appeared at the 

 poles and gradually grew towards the middle of the spindle : (2) that there 

 is still considerable doubt as to how the vesicular bodies formed out of the 

 nuclear plate reconstitute themselves into a nucleus. 



The layer of clear protoplasm around the nucleus during its division has 

 its homologue in the case of the division of the nuclei of the salamander, 

 and the rays starting from this are also found. Klein has suggested that the 

 extra-nuclear rays of the stars around the poles of the nucleus are derived 

 from a metamorphosis of the extra-nuclear reticulum, which he believes 

 to be continuous with the intra-nuclear reticulum. 



The delicate connective filaments usually visible between the two halves 

 of the nuclear plate would seem from Strasburger's latest observations 

 (No. 104) to be derived from the nuclear substance between the striae of the 

 spindle, and to become eventually reabsorbed into the newly-formed nuclei. 



We are it appears to me still in complete ignorance as to the 

 physical causes of segmentation. The view that the nucleus is 

 a single centre of attraction, and that by its division the centre of 

 attraction becomes double and thereby causes division, appears to 

 be quite untenable. The description already given of the pheno- 

 mena of segmentation is in itself sufficient to refute this view. 



