STRUCTURE AND PHYSIOLOGY OF ANIMALS 



21 



it is termed, being the formation of a cup the gastrula (Fig. 7) 



with a double wall. The cavity of the cup-shaped gastrula is the 



arckenteron or primitive digestive cavity ; the opening is termed the 



llastopore, the outer layer of the wall of the 



cup is the ectoderm (or epiblast), the inner 



the cndoderm (or Jiypoblast). The ectoderm 



and endoderm are the primary germinal lay< ,* 



of the embryo ; from one or both of them are 



developed the 1 cells of a third layer the 



mesoderm (mesdblast) which is subsequently 



formed between them. 



This mode of formation of the primary 

 germinal layers in holoblastic oosperms by a 

 process of gastrulation prevails in a number 

 of different sections of the animal kingdom. 

 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 gastru- 

 lation. 



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 de- 

 velopment. 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 arrange- 

 ment of their constituent cells is undergoing gradual modifica- 

 tions, the nature of which is different in the case of different parts. 

 A differentiation of the cells is going on in the developing organs, 

 resulting in the formation of a variety of different kinds of 

 tissues. 



FK ;. 7. Gastmla in longi- 

 tudinal section. a, 

 mouth ; 7', enteron ; 

 c, endoderm; <7, ecto- 

 derm. (From Gegen- 

 baur's ComparoMve An- 



atOnilf.) 





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 rases 

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

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

 (Fig. 8, ); sometimes there is on each cell a single, relatively 

 long, whip-like cilium, which is then termed a flandlnrn (g, k). 

 Cells provided with cilia are termed ciliated, such as bear Hagella 

 flagellate cells. 



Some tissues are composed entirely of cells. Others, though 

 originating from cells or by the agency of cells, consist in greater 



