1889.] Segmentation of the Ovum. 465 
replaced by a dicentric radiation which marks the end of the 
period of repose and the commencement of the first division 
of the ovum. 
Karyokinesisoftheovum. Segmentation is a process of indi- 
rect cell division, and nowhere are more perfect karyokinetic 
figures to be found than in the segmenting ovum. It is, there- 
fore, advisable to give a general account of the changes in- 
volved in every division, but inasmuch as karyokinesis is a 
phenomenon by no means restricted to embryonic cells, it is 
not one of the special subjects of the embryologist. I shall, 
therefore, attempt only a summary account, following in the 
main, O. Hertwig, 26, 37-38, (compare Rabl's exhaustive 
memoir, 37.) 
It is probable that the resting nucleus has one pole at which 
the connection between the reticulum of the nucleus and the 
surrounding protoplasm is more intimate than elsewhere, as 
suggested by Rabl, 38. This pole is marked by a clearer spot 
outside the nucleus, close against it and much smaller than it. 
This clear spot becomes the centre of the radiating arrangement 
of the protoplasm. It was, I believe, first observed by Flem- 
ming in the eggs of Echinoderms, has been seen in Ascaris 
^negalocephala by van Beneden and Neyt, 12, and by Boveri, 
1»> in Siredon by Kolliker, 28, and in other cases. It is now 
designated as the sphere of attraction,' and is seen, at least in 
certain phases, to contain a separate central body (centrosoma 
of Boveri). It is not improbable that the " sphere of attrac- 
tion " is identical with the Nebenkern of recent German writers. 
In a number of instances a small part of the nucleus is seen to 
separate off and to lie as a distinct body, Nebenkern, alongside 
the nucleus; this body has a colorable portion which is com- 
parable to the '• centrosoma." For an account of the scattered 
observations on the Nebenkern, together with the relation of 
these bodies to Gaule's so-called cytozoa, see G. Platner, 34, 
^ The history and significance of the spheres of attraction as here presented 
cannot, by any means, be regarded as final. The observations are few, and in most 
vestigators whatsoever. 
