THE DEVELOPMENT OF ORGANISMS 749 



It may be of interest to note that soon after isolation the blasto- 

 meres frequently explode with violence into minute granules, a fact 

 which suggests the high tension of the cells. 



A general result of importance is that the isolated blastomeres 

 undergo a cleavage that approximates more or less nearly to that 

 of a normal ovum, but the extent of divergence is nearly proportional 

 to the age of the initial form. 



"Even a slight displacement of the blastomeres in the 2-celled 

 stage causes a change in the form of cleavage, such that the blasto- 

 meres of the half-embryo cannot be identified individually with 

 those of a normal embryo half. The normal embryo develops as a 

 unit ; if it be disturbed in the 2-celled stage this unity is destroyed, 

 and two new units are established." 



"The unity of the normal embryo is not caused by a mere 

 juxtaposition of cells"; thus only one of the blastomeres of the 

 2-celled stage may develop, e.g. into a gastrula, the unsegmented 

 blastomere remaining attached. "The unity is not mechanical, but 

 physiological ; there must be a structural continuity from cell to cell 

 that is the medium of co-ordination, and that is broken by 

 mechanical displacements of the blastomeres." 



Lastly, as regards concrete results, it should be noted that "all 

 the induced forms of development show in general a lack of develop- 

 mental power, which becomes more pronounced as the ontogeny 

 advances". 



(4) Pressure Experiments. — Deformations of embryos may be 

 afterwards righted by regulative influences, but teratogenic condi- 

 tions, which are past mending, may also be induced by artificial 

 changes of pressure. 



(5) Radiations. — Many different kinds of radiations have been 

 shown to be influential. Thus increased temperature may induce 

 twinning and increased illumination may cause the albino larvae 

 of Proteus to develop pigment. Some radium rays are lethal, while 

 others provoke variations. 



(6) Chemical Stimuli. — Changes in the chemical medium may 

 have all sorts of influences, from the production of monstrosities to a 

 quickening of normal development or a provocation of new departures. 



(7) Grafting. — Very interesting results have rewarded grafting 

 experiments, such as the evocation of a lens when a piece of tadpole's 

 optic vesicle is cut off and re-implanted under the skin of the side 

 of the body. 



SPEMANN'S IDEA OF "ORGANISERS."— One of the new ideas 

 that has rewarded grafting experiments on embryos is Spemann's 

 idea of "organisers". To explain this, it is necessary to go back to 

 a little elementary embryology — the story of the early stages in the 

 development of the frog. 



