Organisms from Eggs 143 



CramptonS Kofoid, and Conklin that the eggs of 

 right-wound snails do not segment in a symmetrical, 

 but in a spiral, order, and that in left-handed snails 

 the direction of the spiral segmentation is the reverse 

 of that of the segmentation in the right-handed snails. 

 Conklin was able to show that the asymmetrical spiral 

 structure is already preformed in the egg before cleav- 

 age. The asymmetry of the body in snails is therefore 

 already preformed in the egg. ^ 



E. B. Wilson 3 has found a marked differentiation in 

 the eggs of some annelids and molluscs. He isolated 

 the first two blastomeres of the egg of Lanice, an Anne- 

 lid. These two blastomeres are somewhat different 

 in size; from the larger one of the first two blastomeres, 

 the segmented trunk of the worm originates. Wilson 

 found that 



when either cell of the two-cell stage is destroyed, the re- 

 maining cell segments as if it still formed a part of an entire 

 embryo."* The later development of the two cells differs 

 in an essential respect, and in accordance with what we 

 should expect from a study of the normal development. 

 The posterior cell develops into a segmented larva with a 

 prototroch, an asymmetrical pre-trochal or head region, and 



* Crampton, H. E., New York Academy of Sciences ^ 1894; Kofoid, 

 C. A., Proc. Am. Acad. Arts and Sciences, 1894, xxix. 



* Conklin, E. G., Anat. Anzeig., 1903, xxiii., 577; Heredity and 

 Environment in the Development of Man. Princeton, 1915, p. 171. 



'Wilson, E. B., Science, 1904, xx., 748; Jour. Exper. Zool., 1904, 1., 

 I, 197. 



4 The reader will notice the absence of "regulation." 



