DEVELOPMENT OF THE GENERATIVE ORGANS. 717 



Although no axiom in physiology is now better supported 

 than Virchow's aphorism, even Strieker, in 1870, said, ' The 

 formation of fresh nuclei within cells must be admitted to pro- 

 ceed not only from the fission of old nuclei but from the growth 

 of entirely new ones.' Such a belief is, however, now entirely 

 superseded, as the observations of Volkmann and Steudener, 

 Metschnikoff, and others have repeatedly shown that such 

 appearances are entirely deceptive, and due to the invasion of 

 leucocytes, whilst the researches of Flemming, Strasburger, and 

 others have placed the question of nuclear division upon an 

 entirely new footing. Nevertheless, writers on the embryology 

 of Insects are very prone to return to views which are substan- 

 tially the same as those originally held by Schwann, and to 

 adopt the hypothesis that nuclei may originate in a blastema, 

 whilst others maintain that nuclei may be developed from pre- 

 existing nuclei, and exist as such in a blastema, or formative 

 material foreign to that of the cell in which they are formed, 

 and that new cells are developed by the aggregation of this 

 material around such originally naked nuclei. Thus Brandt, in 

 1878 [330], said a general investigation of the manner in which 

 the blastoderm is formed, in Insects, is not unnecessary at the 

 present time, as several conflicting views are held which 

 culminate in two extremes ; according to one of these the 

 cells of the blastoderm arise spontaneously in a peculiar 

 peripheral layer of the yelk --the yelk blastema; according 

 to the other, the cell-substance only is derived from this 

 layer, and the nuclei arise from the division of the germinal 

 vesicle. 



The result of Brandt's investigations were the following con- 

 clusions : the germinal vesicle does not disappear in the yelk, 

 but is itself a cell in other words, a germ-cell, the nucleus of 

 which is the true germinal vesicle that the division of this 

 germ-ovum gives rise to the cells from which the blastoderm is 

 developed. 



So far Brandt, therefore, arrived at the same conclusion as 

 myself; but there is this difference, Brandt apparently believed 

 that the products of the segmentation of the germ-ovum are 



