Embryonic Layers and Membranes 91 



outer layer {ectoderm), while the invaginated cells 

 spread out to form an inner layer [etidoblast) (fig. 

 58). This stage is called the gastrula, and is 

 of great importance, as it recalls the structure of 

 the most primitive group of complex animals — the 

 hydroids and corals (Coelenterates) which exhibit 

 throughout life the simple plan of a hollow sac 

 whose wall consists of two cellular layers. 



Embryonic Membranes. — Around the edge of 



K^ ^ 



Fig. 59. — Diagram showing formation of embryonic membranes. A. amnion; 

 ^. serosa ; B. blastoderm. Magnified. From Nussbaum, in Miall & Denny's 

 " Cockroach." 



the primitive streak, the ordinary, unthickened blasto- 

 derm becomes raised in a double fold. This fold 

 grows over the surface of the primitive streak until 

 its converging edges meet. Its outer layer and its 

 inner layer each become continuous, so that the de- 

 veloping embryo, now sunk into the yolk, is protected 

 by two membranes internal to the yolk-membrane. 

 The outer of these (continuous with the blastoderm) 

 is known as the serosa, the inner ('continuous with the 

 ectoderm of the primitive streak) as the amtiioti (fig. 

 59). But in some insects — Dragonflies, Grasshoppers, 

 Moths, some Beetles, and Bugs — there is an inpushing 



