134 Organisms from Eggs 



skeleton by the formation of crystals consisting of the 

 CaC03by the mesenchyme cells surrounding the intes- 

 tine. For the establishment of the principle in which 

 we are interested the description of morphogenesis 

 need not be carried farther. 



This principle which is under discussion here is the 

 development of a purposeful arrangement of organs 

 out of the egg. If we assume that the egg consists 

 of homogeneous material we are indeed confronted with 

 a riddle. Since the facts contradict such an assump- 

 tion but show, as Boveri has pointed out, a prearrange- 

 ment which allows us to indicate in the unfertilized 

 egg already the exact spot where the intestine will grow 

 into the blastula cavity, we are on solid physicochemical 

 ground, although many questions of detail cannot yet 

 be answered. Such a preformation as Boveri has de- 

 monstrated is only conceivable if the material of the 

 egg has not too high a degree of fluidity; we may con- 

 sider it as consisting essentially of a semi-solid gel 

 which is not homogeneous throughout the egg but 

 divided into three strata. 



2. Lyon^ tried to ascertain whether by centrifuging 

 the sea-urchin egg it was possible to modify its struc- 

 ture and thereby affect the later embryo. He and 

 subsequent experimenters found that it only is pos- 



* Lyon, E. P., Arch. f. Entwcklngsmech., 1907, xxiii., 151; Morgan, 

 T. H., and Spooner, G. B., ibid., 1909, xxviii., 104; Morgan, Jour. 

 Exper. Zool., 1910, ix., 594; Conklin, E. G., ibid., 1910, ix., 417; Lillie, 

 F. R., Biol. Bull., 1909, xvi., 54. 



