THE WHENCE AND THE WHITHEll OF MAN 



these quarters into an upper and lower octant. The 

 cells keep on dividing rapidly , the eight form sixteen, 

 then thirty-two, etc. The sharp angle by which the 

 cells met at the centre has become rounded off, and 

 has left a little space, the segmentation cavity, filled 

 with fluid in the middle of the embryo. The cells con- 

 tinue to press or be crowded away from the centre and 

 form a layer one cell deep on the surface of the sphere. 



This embryo, resembling a 

 hollow rubber ball filled 

 with fluid, is called a blasto- 

 sphere. It corresponds in 

 structure with the fully de- 

 veloped volvox, except, of 

 course, in lacking repro- 

 ductive cells. 



If the rubber ball has a 

 hole in it so that I can 

 squeeze out the water, I can 

 thrust the one-half into the 

 other, and change the ball 

 into a double - walled cup. 

 A similar change takes place 

 in the embryo. The cells of the lower half of the blasto- 

 sphere are slightly larger than those of the upper half. 

 This lower hemisphere flattens and then thrusts itself, 

 or is invaginated, into the upper hemisphere of smaller 

 cells and forms its lining. This cup-shaped embryo is 

 called the gastrula. The cup deepens somewhat and 

 becomes ovoid. Take a boiled egg, make a hole in 

 the smaller end and remove the yolk, and you have a 

 passable model of a gastrula. The shell corresponds 

 to the ectoderm or outer layer of smaller cells ; the 



4. GASTRUZA. HATSCHEK, FROM 

 HERTWIG. 



Outer layer is the ectoderm ; inner 

 layer, the entoclerm ; internal 

 cavity, the archeuteron ; moiith 

 of cavity, blastopore. 



