THE DEVELOPMENT OF ORGANISMS 747 



Formstorung (1880), in which the results of abnormal disturbances 

 were again utilised to throw light on the causes of normal develop- 

 ment, showing how artificial deforming might aid in the understanding 

 of normal forming. We must also refer to the important work 

 of W. His {Unsere Korperform). Thus he sought to show how the 

 amount and disposition of yolk, the pressure of membranes, the 

 mutual influences of parts, and the environmental stimuli were 

 factors in causing foldings and .pouchings, ingrowths and out- 

 growths, and other processes of development. Thus, to take a 

 simple case, if there is a considerable amount of yolk in an egg-cell, 

 as in the frog's, it will sink to the lower hemisphere, and will there 

 inhibit the rate of egg-cleavage. The resulting ball of cells will show 

 an upper hemisphere of numerous small cells, and a lower hemisphere 

 of a few large yolk-laden cells. The continuation of this inequality 

 of growth will necessarily lead to an overgrowth on the part of the 

 smaller cells. 



The work of His was an attempt to correlate the potentialities 

 of the embryo with its conditions of development, and thus rationalise 

 each advancing phase of growth and change : an obviously sound and 

 necessary inquiry, illuminating the life-process. Though this was not 

 at first appreciated by previous observers (since their evolutionary 

 interest was too purely morphological). His was soon the inspirer of 

 workers activated by this physiological view. Rauber's contribution 

 was to corroborate the physiological interpretation of the normal 

 course of development by data derived from teratogenic (i.e. mon- 

 strosity-making) experiments. 



(2) Puncturing Experiments. — ^When Roux punctured one of 

 the first two segmentation cells into which a frog's egg divides, 

 he found that the intact other cell developed into a half-ornbiyo. 

 Thus it might show half of the normal cerebrum, one ear-sac, a 

 one-sided gut, and so on. But when Oscar Hertwig destroyed one 

 of the first two segmentation-cells, the surviving half formed a 

 fairly normal embryo of dwarf size, but not a A«//-embryo. Observers 

 of equal competence working with the same material reached 

 discrepant results. But T. H. Morgan proceeded to show that either 

 a half-embvyo or a whole half-sized dwarf may result from the 

 experiment, according to whether the uninjured blast omere was 

 kept in a fixed position (Roux), or allowed to move freely in the 

 water (Hertwig), so that rearrangement of egg-materials was 

 effected. 



(3) Isolation Experiments. — Some striking results have been 

 obtained by shaking apart or otherwise isolating the segmentation- 

 cells into which an egg-cell divides, and no instance is more instruc- 

 tive than the work of E. B. Wilson on the developing ova of the 

 lancelet or Amphioxus. 



The early stages in the development of Amphioxus are as interest- 



