196 DYNAMICS OF LIVING MATTER 



more viscous and jellylike, keeps the hemispherical shape, even after 

 its isolation. If this idea is correct, it ought to be much more difficult 

 to produce a giant embryo by the fusion of the contents of two eggs, 

 in Ctenophores than in sea urchins, inasmuch as their fusion would be 

 more difficult on account of the jellylike consistency of the main mass 

 of the embryo. 



It seems that in the egg of Mollusks, also, the simple symmetry 

 relations of the body are already preformed. It is well known that 

 there are shells of snails which turn to the right while others turn in 

 the opposite direction. The shells of Lymnceus turn to the right, those 

 of Planorbis to the left. It had been observed that the eggs of right- 

 wound snails do not segment in a symmetrical, but in a spiral, order. 

 Crampton and Kofoid discovered independently of each other that 

 in left-handed snails an asymmetrical spiral segmentation occurs also, 

 but the direction of the spiral is the reverse of that in the segmentation 

 of the right-handed snails.* The asymmetry of the body in snails is 

 therefore already preformed in the egg. The conditions which de- 

 termine such an asymmetry may be of a very simple character-! 



From the facts we have thus far discussed it is obvious that in eggs 

 whose contents possess a high degree of fluidity not much beyond the 

 simplest symmetry relations can be preformed. A higher degree of 

 preformation is only possible where liquid and solid constituents are 

 contained in different parts of the egg. 



E. B. Wilson { has recently found a still more marked differentiation 

 in the eggs of some Annelids and Mollusks than the cases thus far 

 discussed. Wilson isolated the first two blastomeres of the egg of 

 Lanice, an Annelid. 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 remaining 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 pos- 

 terior cell develops into a segmented larva with a prototroch, an asym- 

 metrical pre-trochal or head region, and a nearly typical metameric 

 seta-bearing trunk region, the active movements of which show that 

 the muscles are normally developed. The pre-trochal or head region 

 bears an apical organ, but is more or less asymmetrical, and, in every 

 case observed, but a single eye was present, whereas the normal larva 



* Crampton, New York Academy of Sciences, 1894; Kofoid, Proceedings of the Am. 

 Academy of Arts and Sciences, Vol. 29, 1894. 



t Conklin, Anatomischer Anzei^er, Vol. 23, p. 577, 1903. 



j E. B. Wilson, Science, Vol. 20, p. 748, 1904; and Jour. Exper. Zool., Vol. I. 



