CHAP. XII The Life-History of Animals 195 



the first division of the egg-cell have been shown to consist 

 in equal proportions of material derived from the male- 

 nucleus and from the ovum-nucleus. 



Yet in the last century naturalists still spoke of an " aura 

 seminalis," and believed that a mere breath, as it were, of 

 the male cell was sufficient to fertilise an egg, and it was 

 only in 1843 that Martin Barry discerned the presence of 

 the spermatozoon within the ovum. 



8. Segmentation and Development.^ — The fertilised 

 egg-cell divides, and by repeated division and growth of 

 cells every embryo, of herb and tree, of bird and beast, is 

 formed. On the quantity and arrangement of the yolk the 

 character of the segmentation depends. When there is 

 little or no yolk the whole ovum divides into equal parts, as 

 in sponge, earthworm, starfish, lancelet, and higher mammal. 

 When there is more than a little yolk, and when this sinks 

 to the lower part of the egg-cell, the division is complete 

 but unequal, and this may be readily seen by examining 

 freshly-laid frog spawn. When the yolk is accumulated in 

 the core of the egg-cell, the more vital superficial part 

 divides, as in insects and crustaceans. Lastly, when the 

 yolk is present in large quantity as in the ova of gristly 

 fishes, reptiles, and birds, the division is very partial, being 

 confined to a small but rapidly extending area of formative 

 living matter, which lies like a drop on the surface of the 

 yolk. 



As the result of continued division, a ball of cells is 

 formed. This may be hollow (a blastosphere), or solid 

 (a morula, i.e. like a mulberry), or it may be much modi- 

 fied in form by the presence of a large quantity of yolk. 

 Thus in the hen's egg what is first formed is a disc of cells 

 technically called the blastoderm, which gradually spreads 

 around the yolk. 



The hollow ball of cells almost always becomes dimpled 

 in or invaginated, as an india-rubber ball with a hole in it 

 might be pressed into a cup-like form. The dimpling is the 

 result of inequaUties of growth. The two-layered sac of cells 

 which results is called a gasirula, and the cavity of this sac 

 becomes in the adult organism the digestive part of the 



