TROPHOBLAST AND SEROSA. 599 



The above are actual instances of ontogenetic substitution. 

 Instances of phylogenetic substitution can^ in the nature of 

 things, only be hypotlietical. 



In the embryos of the Poduridse there is no amnion (Uljanin 

 [10], Wheeler [12] ), and therefore no serosa. The blastoderm, 

 however, is not entirely employed in forming the dorsal body 

 wall of the embryo, since there is a dorsal organ which is some- 

 times spoken of under the name micropylar organ (Wheeler) 

 and spherical organ (Uljanin). This dorsal organ is a local 

 thickening of the blastoderm lying in front of the embryonic 

 tract ; but at a later stage, when the true topographical rela- 

 tions are established, it is found to lie in the nuchal region (cf. 

 Lemoine's figures [8] ). It is destined to be absorbed like the 

 dorsal organ of higher insects, which is the product of the 

 involution of the serosa. 



Appearances are in favour of the Poduridae never having 

 had an amnion, but the embryos present the remarkable pecu- 

 liarity that the blastoderm outgrows the vitellus to 

 such an extent that it is thrown into numerous complicated 

 folds at the surface of the egg. " La superficie de la couche 

 blastodermique," says Uljanin (10, p. xvii), " devient de plus 

 en plus inegale ; cette superficie, beaucoup plus agrandie k 

 cause des inegalites de la couche blastodermique, secrete une 

 membrane cuticulaire, cuticule blastodermique, que Ton 

 voit d travers le chorion de I'oeuf fortement plissee et suivant 

 toutes les inegalites du blastoderme epaissi." Lemoine (8) 

 figures eggs with such a folded blastoderm. 



The fact that the blastoderm of Poduridae out- 

 grows the vitellus, involuntarily suggests an ata- 

 vistic repetition of a state in which the size of the 

 egg bore no relation whatever to the dimensions of 

 the blastodermic vesicle. 



The indusium of Locustid embryos is a remarkable structure 

 discovered by Wheeler (12). It arises as a small circular 

 local thickening of the serosa immediately in front of the head 

 of the embryo, i. e. in a position corresponding to the nuchal 



