FORMATION OF FIRST RUDIMENTS OF EMBRYO 531 



in height, and now form a cubical or cylinder epithelium, which 

 continuously envelops the surface of the egg. (Korschelt and 

 Heider.) 



e. Formation of the first rudiments of the embryo, and of the 



embryonic membranes 



The embryo first arises as a whitish streak or band-like thickening 

 on the ventral side of the egg, and is variously called the " primitive 

 streak," "primitive band," "germinal band," or " embryonal streak." 

 In most cases the primitive band is divided at regular intervals by 



C 



D 



FIG. 507. Formation of the blastoderm in Hydrophilus : b, completed blastoderm ; <7, yolk ; /, 

 so-called division-cells ; /t, " keimhautblastem. " ; ", yolk-cells. After Heider, from Korschelt and 

 Heider. 



transverse furrows, indicating the limits of what are to be the body 

 segments. 



Cross-sections (Fig. 509) show that the band is composed of 

 several layers, i.e. an outer layer (ectoderm) and an inner layer 

 which comprises the endoderm and mesoderm, and so long as these 

 two layers are not sharply differentiated from one another, this 

 second layer may be called, with Kowalevsky, "the inner lower 

 layer, or ento-mesoderm " (Figs. 508, 509, B, C, ). 



It is characteristic of insects, only rarely occurring in other 

 arthropods (e.g. the scorpion), that the primitive streak is not situ- 

 ated on the surface of the egg, but becomes overgrown by a folded 

 structure (Fig. 508, a/) rising from its edges, the amnion-fold, so 



