EXPERIMENTAL MORPHOGENESIS 97 



require to be described here a little more fully. All these 

 researches, which have been carried out almost exclusively 

 by Herbst, 1 relate to the effect of the chemical components 

 of sea-water upon the development of the sea-urchin. If 

 we select the most important of Herbst's results, we must 

 in the first place say a few words on the part taken by 

 lime or calcium, not only in establishing specific features of 

 form, but in rendering individual morphogenesis possible at 

 all. Herbst has found that in sea-water which is deprived 

 of calcium the cleavage cells and many tissue cells also 

 completely lose contact with each other : cleavage goes on 

 quite well, but after each single division the elements are 

 separated ; at the end of the process you find the 808 cells 

 of the germ together at the bottom of the dish, all swim- 

 ming about like infusoria. There seems to be some 

 influence of the calcium salts upon the physical state of 

 the surfaces of the blastomeres. 



It is not without interest to note that this discovery 

 has an important bearing on the technical side of all experi- 

 ments dealing with the isolation of blastomeres. Since the 

 separation of the single cleavage elements ceases as soon 

 as the germs are brought back from the mixture without 

 lime into normal sea-water, it of course is possible to 

 separate them up to any stage which it is desired to study, 

 and to keep them together afterwards. Thus, if for instance 

 you want to study the development of isolated cells of the 

 eight-cell stage, you will leave the egg in the artificial 

 mixture containing no calcium until the third cleavage, 

 which leads from the four- to the eight-cell stage, is finished. 

 The single eight cells brought back to normal sea-water at 



1 Arch. Entw. Mech. 17, 1904. 



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