148 THE BIOLOGY OF AN ANIMAL. 



surround a central cavity known as the segmentation-cavity. 

 This cavity increases in size in later stages, so that the embryo 

 finally appears as a hollow sphere surrounded by a wall consist- 

 ing of a single layer of cells. This stage is known as the llastula 

 (or Hastosphere) (a, 5, Fig. 84). 



Formation of the Germ-Layers. The blastula still consists of 

 nndifferentiated cells, and has no organs. From this time for- 

 wards the whole course of development is a process of differ- 

 entiation, both of the cells and of the organs into which they 

 soon arrange themselves. The first step in this process is a flat- 

 tening of the embryo at the lower pole i.e., the half consisting 

 of larger cells (Fig. 84, c). The large cells are then folded into 

 the segmentation-cavity so as to form a pouch opening to the ex- 

 terior ; at the same time the embryo becomes somewhat elon- 

 gated. 



This process is known as invagination, and at its completion 

 the embryo is called the gastrula. The infolded pouch (called 

 the archenterori) is the future alimentary canal ; its opening (now 

 known as the Uastopore) will become the mouth ; and the layer 

 of small cells over the outside will form the skin or outer layer 



*/ 



of the body-wall. 



The embryo very soon begins to swallow, through the blasto- 

 pore, the milk-like fluid in which it floats, and to digest it within 

 the cavity of the archenteron. 



It is obvious that the embryo already shows a distinct differ- 

 entiation of parts which perform unlike functions. In fact we 

 may regard the gastrula as composed of two tissues still nearly 

 similar in structure but very unlike in function. One of these 

 consists of the layer of cells which forms the outer covering ; 

 this tissue is known as the ectoblast (ec, Fig. 84). The second 

 tissue is the layer of cells forming the wall of the archenteron ; 

 it is called the entoblast (en). The ectoblast and entoblast to- 

 gether are known as germ-layers. 



Meanwhile changes are taking place which result in the for- 

 mation of a third germ-layer lying in the segmentation-cavity be- 

 tween the ectoblast and entoblast and therefore called the meso- 

 l>last (;, Figs. 84, 85). As the entoblast is invaginated two large 

 cells separate from it and pass into the segmentation-cavity (Fig. 

 84, c). These are known as primary mcsoblastic cell* because 

 they afterwards give rise to the mesoblast. Each of these cells 



