CHAPTER XV 



DEVELOPMENT 



1. Segmentation and after 2. Differentiation 3. Some generalisa- 

 tions : (a) the ovum theory ; (&) the gastrsea theory ; (c) 

 recapitulation ; (d) organic continuity. 



1. Segmentation and After.- -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 in part depends, but 

 there are other factors involved. 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 many 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 volk. 



\j 



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. 



313 



