REGENERATION IN EGG AND EMBRYO 22Q 



Hertwig's method of shaking the eggs, and studied the development 

 of the isolated blastomeres. He found that the cleavage was strictly 

 that of a half, and not like that of a whole egg. The normal egg 

 divides into two, four, and eight equal parts. At the next division, four 

 of the cells divide very unequally, producing four very small cells, 

 the micromeres, at one pole. The four cells of the other hemisphere 

 divide equally (Fig. 64, /). The isolated blastomere divides at first 

 into two equal parts, then again into equal parts. At the next divi- 

 sion two of the cells produce micromeres and two divide equally (Fig. 

 64, G). This is exactly what happens at this division in each half, if 

 the blastomeres are not separated. In later stages a half-sphere is 

 formed that is equivalent to half of the normal sphere (Fig. 64, C). 

 The open side corresponds to the side at which the half would have 

 been united to the other half. Thus up to this point a half-cleavage 

 and a half-blastula have appeared. 1 



In later stages the open half-blastulae close in, producing a whole 

 sphere that becomes perfectly symmetrical (Fig. 64, D}. A symmetri- 

 cal gastrula develops (Fig. 64, E) by the invagination of a tube at one 

 pole, and a symmetrical embryo is formed (Fig. 64, F) that resembles 

 the normal embryo except in point of size. 



Driesch has also found that a number of twin embryos arise from 

 the shaken eggs. They arise from eggs whose blastomeres have been 

 disturbed or shifted, so that each produces a small whole embryo, the 

 two embryos being united to each other in various ways. 



In a second paper, published in the following year, Driesch ex- 

 tended his experiments, and attempted to discover how far the " inde- 

 pendence " of the blastomeres extends ; i.e. he tried to find out if all 

 the blastomeres resulting from the cleavage are alike. When one 

 of the first four cells is separated from its fellows by shaking, it 

 continues to divide, in most cases as a quarter, and produces later 

 a small spherical blastula. Many of these blastulae, although 

 apparently healthy, never develop further, although they may remain 

 alive for several days. In one experiment only eight out of twenty- 

 six reached the pluteus stage, with a typical digestive tract and 

 skeleton. 



From these experiments Driesch drew the important conclusion 

 that the cleavage cells or blastomeres of the sea-urchin's egg are 

 equivalent, in the sense that if they were interchanged a normal em- 

 bryo would still result. A somewhat similar view is expressed in the 

 dictum that the position of a blastomere in its relation to the others 

 determines what part it will produce, if its position is changed it 

 gives rise to another part, etc., or, expressed more concisely, the 



1 In some cases, especially in sphserechinus, even at the eight-celled stage, the blasto- 

 meres seem to shift their position, so that a whole sphere of half size is formed. 



