EGG SEGMENTATION IN APHIS. 135 



Thus the rudimentary embryonic form may be 

 roughly likened to a tube the walls of which are four- 

 fold. These layers may be mechanically separated 

 from each other. All animals go thus far through 

 this stage in development ; and the steps have been 

 distinctly traced in the higher Insects, including 

 Aphides. 



All ova may be grouped under two divisions, 

 dependent on the manner in which the vitelline cells 

 are arranged. The first group is called Telolecithal, 

 in which the yolk when present is concentrated at the 

 "vegetative pole " of the ovum. In the second group, 

 called Centrolecithal, this food-yolk (Dotters Elemente) 

 is concentrated at the centre of the ovum.* The 

 segmentation of Aphis belongs to this latter division. 



Metschnikoff has shown that in SiphonopTiora rosce 

 the first segmentation-nucleus divides in two, each 

 part of which takes up a position in the clearer peri- 

 pheral protoplasmic layer of the egg. After further 

 subdivision, the nuclei, which are enveloped in proto- 

 plasm, arrange themselves so as to form a peripheral 

 blastoderm composed of columnar cells. f 



Brandt divides Hemipterous (Homoptera) Insects 

 into two great groups dependent on the differentiation 

 of their respective ova ; but too much stress should not 

 be given to classification on mere segmentation. 



Homoptera. 



Without yolk-elements. With yolk-elements, 



(mit panoistichen Eirohren) (mit meroistichen Eirohren) 



Psylla, Aphida? foam., Coccus, 

 Aphidce neut. Pediculus. 



Prof. Moseleyf has recently produced an interesting 

 memoir on the habits and morphology of Peripatus 

 capensis, a Julus-like animal, found under rotten 



* Balfour, I. c, vol. i, p. 100 ; also see Brandt, ' Ueber das Ei,' p. 48. 



t Metschnikoff on " Aphis rosae," ' Emb. Stud. Insecten,' Band xvi. 

 Vide my PI. H, fig. 12. 



t Prof. H. N.Moseley " On Peripatus," 'Phil. Trans.,' 1874, vol. 164, 

 p. 757. 



