I STRUCTURE AND PHYSIOLOGY OF ANIMALS 23 



sphere, the Nastosphere or Uastula, with a wall composed of a 

 single layer of cells enclosing a cavity— the segmentation cavity 

 or hlastocmh. 



One side of the hollow blastula next becomes pushed inwards or 

 invaginated (Fig. 9, B, C), as one might push in one side of a hollow 

 india-rubber ball, the result of this process of invagination, or 

 gastndation as it is termed, being the formation of a cup— the 

 gastrula (Fig. 10)— with a double wall. The 

 cavity of the cup-shaped gastrula is the 

 archenteron or primitive digestive cavity ; 

 the opening is termed the Uastopore, the 

 outer layer of the wall of the cup is the 

 ectoderm (or epiUast), the inner the endoderm 

 (or hypoblast). The ectoderm and endoderm 

 are the primary germinal layers of the em- 

 bryo ; from one or both of them are developed 

 the cells of a third layer — the mesoderm 

 (mesoUast) — which is subsequently formed 

 between them. 



This mode of _ formation of the primary '^'"■tuSnar^sSionr^i 

 germinal layers in holoblastic oosperms bv a blastopore ; 6, aroh- 



f i_ -t ■ • •!• 1 enteron ; c, endoderm ; 



process oi gastrulation prevails in a number a, ectoderm. (From 



of different sections of the animal kingdom. Sn^'^^if "*''™- 



In many animals, however, it becomes modi- 

 fied or disguised in various ways; and in many meroblastic 

 oosperms it is doubtful if there occurs anything of the nature of 

 true gastrulation. 



The cells of the three germinal layers give rise to the various 

 organs of the body of the fully-formed animal — each layer having 

 a special part to play in the history of the development. As the 

 various parts of the embryo become gradually moulded from the 

 cells of the germinal layers, it becomes evident on comparison 

 that their internal structure — the form and arrangement of their 

 constituent cells — is undergoing gradual modifications, the nature 

 of which is different in the case of different parts. A differentia- 

 tion of the cells is going on in the developing organs, resulting in 

 the formation of a variety of different kinds of tissues. 



4. Tissues. 



The cells of the tissues of the animal body differ greatly in 

 form in different cases. Some are rounded, others cubical, others 

 polygonal ; some are shaped like a pyramid, others like a cone, 

 others like a column or cylinder ; others are flattened and tabular 

 or scale-like. Cells situated on free surfaces are in many cases 

 beset at their free ends with delicate, hair-like structures or cilia 

 which vibrate to and fro incessantly during the life of the cell 



