134 BRITISn APHIDES. 



cheata, though it does so arise in the Crustacea. Prof. 

 Balfour also states that it is very unlikely so to occur 

 in Myriapoda.* 



Dollinger first used the term Blastodermf, and he 

 early distinguished the three layers under the heads 

 serous, mucous, and vascular. Von Baer studied under 

 Dollinger, and he produced his great work on the evolu- 

 tion of animals during the periods from 1828 to 1837. J 



The membranes of the germinal disc or blastoderm 

 are known by the under-mentioned terms, and out of 

 them the various organs are elaborated. § 



Questions are confessedly difficult to answer, as to 

 the complete or partial homology of these layers, and 

 of the organs proceeding from them; and in parti- 

 cular there appears to be uncertainty as to the exact 

 derivative of the sexual organs of animals. 



In the hydroid polyps, and in the low sponges like 

 Haliphysema, the male and female cells are placed in 

 juxtaposition, and on the same primitive layer, viz. 

 the Entoderm. In many instances amongst higher 

 animals the male organs are derived from the ecto- 

 derm or skin-layer ; whilst the female organs develop 

 from the intestinal layer or entoderm. But yet again 

 there are instances where both the sexual organs seem 

 to arise from the two layers combined. This may be 

 from adaptational causes. 



Ectoderm ^ Skin, sensory organs, including brain, 

 or fa 1 spinal cord, and nerves. 



Epiblast ) *■+! (Male sexual organs ?) 



Mesoderm % Flesh, bones, ligaments. 



Entoderm ") ^ Nourishing organs, including liver, 



or [ ^ lungs, and intestines. 



Hypoblast ) (Female sexual organs ?) || 



* F. M. Balfour, ' Treatise on Comparative Embryology,' vol. i, 

 p. 451. 



f UXaoroc and Slpiia, skin, membrane. 



j Von Baer, ' Entwickelungeschichten dor Thiere.' 



I Vide Plate H, figs. 13 and 14. 



|[ As authority for the above grouping, vide M. Balfour, op. cit., vol. i, 

 p. 103 ; and E. Haeckel, op. cit., vol. ii, pp. 3S9 to 401. 



