150 JOHN PRIESTLEY. 



Strasburger, believe that the germinal vesicle vanishes en- 

 tirely, necessarily suppose the existence of an enuclear stage 

 in the develoj^ment of the first cleavage mass. But on this 

 point Hertwig thinks there is room for doubt. 



The difficulty of distinguishing in the fresh condition a 

 small body like the original germinal spot, with refractive 

 powers but slightly different from those of the protoplasmic 

 mass which surrounds it, has been much underrated. Until, 

 therefore, further attempts to differentiate a nucleus or in- 

 terior body by means of staining fluids have led to similar 

 nesfative results, this criticism must be allowed to have 

 weight. 



'Ihe statements as to the origin of the primary cleavage 

 nucleus are those which present the points of greatest difier- 

 ence among the authors who have recently investigated the 

 matter. Strasburger's view is, however, singular ; for he con- 

 nects the nucleus in question with the skin or cortical 

 layer of protoplasm of the ovum, by direct descent. This 

 he does without putting forward any strong reasons 

 for such a relationship, and in spite of the fact that the 

 nucleus stains much more deeply than any other part of the 

 ovum when treated with colouring fluids — a circumstance 

 which Hertwig suggests as indicating a distinction of sub- 

 stance. 



The rest describe a fusion of pronuclei, generally of two, 

 but sometimes of more than two.^ To each of these pro- 

 nuclei Auerbach ascribes a similar origin, viz. the vacuola- 

 tion of the cell-body and the collection into the vacuoles of a 

 fluid nuclear matter. Both Hertwig and van Beneden speak 

 of the pronuclei as bodies staining in colouring fluids, and 

 having other nuclear characteristics ; and both regard one of 

 the pronuclei (Hertwig's Spermakei'n, van Beneden's pro- 

 nucleus peripherique) as possibly in some way connected with 

 the fertilising element, — indeed, Hertwig believes it to be 

 the head or nucleus of the spermatozoid. 



In the subsequent history of the nucleus, as has already 

 been said, there is greater agreement. The discrepancies 

 between the account of Auerbach and those of Strasburger, 

 Hertwig, and Van Beneden, or, more correctly speaking, the 

 incompleteness of the former as compared with the latter, is 

 entirely explained by the respective methods of observation 

 adopted; since HertAvig, when he examined recent specimens 

 only, overlooked the interior nuclear changes so fully verified 

 by Strasburger — an oversight which he repaired by a study of 



1 Biitschli. 



