120 THE BIOLOGY OF THE ANIMAL 



which after a time will push out from the cen- 

 tre of the sphere, thus forming a hollow sphere, 

 the cavity now enclosed by the layer of cells 

 being known as the segmentation-cavity. 

 When the egg has reached this stage of develop- 

 ment it may require careful focussing to show 

 that the sphere is really hollow. Does the mem- 

 brane still surround the egg? Has the egg 

 increased in size? Can you detect any move- 

 ment of the egg as a whole or of the sphere 

 inside the egg membrane ? 

 Draw a blastula as seen from the surface and in op- 

 tical section, *. e., as though one-half the sphere were 

 removed, leaving the cavity visible. 



5. The gastrula. — At the end of about twenty- 

 four hours after fertilization, or perhaps soon- 

 er, it will be noticed that at a certain point 

 the surface of the blastula becomes indented as 

 though being pushed in (invagination). The 

 depression increases, and the invaginated mass 

 of cells encroaches upon the segmentation cav- 

 ity. The depression is the primitive mouth or 

 digestive cavity of the embryo or gastrula, 

 which now consists of two layers of cells, the 

 original outer layer, or ectoderm, and the lay- 

 er of invaginated cells, or entoderm. Between 

 these layers lies the segmentation cavity, while 

 the entoderm lines the digestive cavity. In 

 this stage the cells of the ectoderm will be 

 seen to be covered with cilia, by means of 

 which the embryo rotates within the egg- 

 membrane. The membrane soon breaks, and 

 the embryo swims about freely in the water. 

 Before this happens it may be noticed that the 



