HEREDITY 



193 



much as they show how twins, triplets, and quadruplets can originate 



from one egg. The lack of any compKcated structure in the unseg- 



mented egg is, I beUeve, evidenced very strikingly in the following 



observation. I have mentioned in a former lecture that if the egg of 



a sea urchin (Arbacia) is put into diluted sfea water (equal parts of 



sea water and distilled water), many eggs 



will burst, and part of the protoplasm will 



flow out, without necessarily being separated 



from the rest of the egg. In this case the 



normally spherical egg is transformed into a 



double sphere or a dumb-bell-shaped mass 



(Figs. 35 and 36). This mass may give 



rise to a single embryo, or to "Siamese 



twins," and whether the one or the other 



occurs depends upon the width of the piece 



ab (see Figs. 35 and 36) that connects the two spheres.* If this 



piece is very narrow, as in Fig. 36, twin blastulae will originate from 



such an egg; if it is wide, as in Fig. 35, 

 only a single embryo will develop from 

 it. Why this should be so can be readily 

 recognized. We have already stated that 

 the cells have a tendency to creep to the 

 periphery of the egg, thus leaving an 

 empty space in the center which becomes 

 the blastula cavity. When the connecting 

 piece is very narrow, it will be filled with 

 cells, and the two segmentation cavities 



can and will remain separate, and two blastulae will be formed (Fig. 



37). If, however, the piece ab is wide (Fig. 38), an open space will 



be left in this connecting piece, by which 



the two blastula cavities communicate, 



and in this case only one blastula cavity, 



and hence only one embryo will be formed. 



The distorted dumb-bell-shaped blastula 



soon becomes spherical (through the 



secretion under pressure of Hquid into 



the interior), and a normal larva results. 



These facts prove that as far as the 



formation of the blastula is concerned there is no preformed structure of 



any high degree of complication present in the egg; and this is still more 



true for the later embryonic formations, which follow the blastula stage. 

 * Loeb, Archivfur Entwickelungsmechanik, Vol. 8, p. 363, 1899. 



HucUus 



Fig. 37. 



