60 BIOLOGY 



Fertilisation, which may be effected in various ways 

 according to the habitat of the organism, is followed by 

 segmentation. On the quantity and arrangement of 

 the yolk in the egg the character of the segmentation 

 depends. When the yolk is present in small amounts 

 the whole egg divides into equal parts, as is seen in 

 sponge, earthworm and star-fish. When there is a 

 considerable quantity of yolk and it has collected at 

 one part, the division is complete but unequal, as we 

 see in the frog's egg. When the yolk is accumulated 

 in the centre there is a superficial division, and when 

 present in large quantity the division is confined to a 

 small but rapidly extending area of living matter which 

 rests on the surface of the yolk. 



The final result of some of the types of segmentation 

 is a ball of cells which may be hollow, and is known as 

 a llastula, or solid, and is known as a morula. It may, 

 however, be modified in various ways by the presence 

 of a large quantity of yolk. 



The hollow ball of cells almost always becomes 

 invaginated ; this is, the lower pole of the egg is pushed 

 up until the cells come into contact with the inner 

 surface of the cells at the other pole. The result of 

 this invagination is the formation of a two-layered sac 

 of cells which is named the gastrula ; its outer layer of 

 cells is known as the epiblast, its inner as the hypoblast, 

 and the cavity as the archenteron, which becomes the 

 digestive part of the food canal in the adult. From the 

 outer layer, the epiblast or ectoderm, are formed in the 

 adult the outer skin, the nervous system, and the most 

 important parts of the sense-organs ; while from the 



