146 



AN INTRODUCTION TO MODERN GENETICS 



tion and the development of endoderm. Embryonic determination is a 

 result of the interaction of these two gradients. Whereas in the verte- 

 brates the pattern of the determined elements is apparently impressed 

 upon them by an active organization centre working on relatively 

 passive competent material, here the pattern is a balance between two 



Fig. 71. Diagram illustrating the Gradients in Echinoderm Eggs. — A 



cleavage stage is shown on the left with the animal pole upwards and vegetative 

 pole downwards. The four main circlets of cells can be separated and allowed to 

 develop in isolation. The middle column shows that the uppermost (animal — 1) 

 forms long sensory cilia over the whole surface, and fails to gastrulate. In animal — 2 

 the sensory cilia are confined to the upper half. Vegetative — 1 usually gives no 

 sensory cilia or gut, but sometimes one or other of these is present in a reduced 

 form. Vegetative — 2 gastrulates and develops an abnormally large gut. If the small 

 cells at the vegetative pole (micromeres) are isolated they fall apart and do not 

 develop. The column on the right shows the normal gastrulae formed from 

 combinations of an. — 1 + 4 micromeres, or an. — 2 + 2 micromeres or veg. — 1 + 1 

 micromere. 



(After Horstadius.) 



active regions. Possibly such a balance, which involves the whole 

 extent of the echinoderm egg, will be found within the organization 

 centre of the vertebrates. 



The two opposing gradients are already present in the unfertilized 

 egg, and are clearly cytoplasmic in nature. Something is already known 

 about their metabolic basis ;^ it is suggested that the animal gradient 

 involves a carbohydrate oxidation which can be inhibited by lithium 

 salts, while the vegetative gradient depends on an anaboHc process, 



^ Lindahl 1936. 



