64 SYDNEY J. HIOKSON. 



all of theirij we have not received any account of the first 

 division of the oosperm nucleus. 



It is quite unreasonable to suppose that all these observers 

 would have overlooked a nuclear division — which we might 

 expect, if it exists at all, to be the largest and most conspicuous 

 of the whole series. Nor can we suppose that the methods of 

 preservation or staining was so consistently bad at the first 

 stage as to prevent the observation of the figure, and so fre- 

 quently good in the later stages as to show the whole process of 

 karyokinesis clearly and distinctly. 



Now Kastschenko (31) shows that in Elasmobranchs a 

 number of nuclei appear in the blastoderm and the surround- 

 ing yolk before the formation of the segmentation furrows, 

 which appear not in regular sequence, but simultaneously and 

 irregularly. " Die bekannte regelmassige Reihenfolge des 

 Erscheinens des Segmentationsfurchen existiert bei Selachiern 

 fast gar nicht. Nur in seltenen Fallen bemerkt man das 

 urspriingliche Erscheinen einer Segmentationsfurche, welcher 

 dann gleichzeitig mehrere andere unregelmassig sich kreuzende 

 folgen. In den meisten Fallen aber erscheinen schon vom 

 Anfang an mehrere Segmentationsfurchen gleichzeitig und 

 somit zerfallt die Keimscheibe direct in mehrere verschieden 

 grosse Segmentationskugeln, welche sich dann weiter aber nicht 

 gleichzeitig theilen." 



We have, then, at the commencement of the development of 

 the Elasmobranch a multinucleated plasmodium, and Kast- 

 schenko is of opinion that all tbe nuclei of this plasmodium are 

 formed by repeated divisions of the first segmentation nucleus- 

 But, like all his predecessors, Kastschenko was apparently 

 unable to observe these repeated divisions of the first nucleus, 

 and it seems extremely probable that in Elasmobranchs, as in 

 insects, Hydrocorallines, and others, we have at this stage a 

 true process of nuclear fragmentation. 



I have already called attention to the fact that in all of these 

 cases in which the fragmentation of the oosperm nucleus 

 probably occurs the ovum does not segment immediately after 

 fertilisation ; that there is, in fact, for a time in the early em- 



