W. K. BEOOKS ON THE GENUS SALPA. 243 



the plane of symmetry, as it does in salpa, although the point where 

 fertilization takes place is not fixed as it is in salpa. 



Hatscheck holds, as is well known (Ueber Entwicklungsgeschichte 

 von Teredo, Arb. Zool. Inst. Wien, 111, 1880, p. 25), that bilateral sym- 

 metry is present potentially in the molecular structure of the egg 

 before fertilization, and that it is simply made manifest as develop- 

 ment advances; but Metschnickoff (Vergleichend-embryologische Stu- 

 dien, 3, Zeit. f. Wiss. Zool. XXXVII, 1882, p. 303) has shown that this 

 interpretation is unwarranted, and Davidoff (16, p. 581) agrees with 

 Metschnickoff that the early appearance of bilateral symmetry in seg- 

 menting eggs must be attributed to secondary modification of the onto- 

 genetic record. 



Every naturalist knows that the bilateral animals have not inherited 

 this characteristic from a common source, for there are abundant 

 examples of the secondary acquisition of bilateral symmetry, as in the 

 spatangoids for example, and in the pulmonate gasteropods, and in 

 the bilateral species of anthomedusaB and in the siphonophores, to say 

 nothing of the infusoria, and no one can believe that it is as old, phylo- 

 genetically, as fertilization or segmentation, and if this is true, as it 

 certainly is, and if its early appearance is due to secondary acceleration, 

 there is no reason why it should come into existence at the same stage 

 in all animals. In some animals it may possibly be potential in the 

 ovarian eggs and may even coincide with the symmetrical plane of the 

 body of the parent, and if this should some day be proved we should still 

 have every reason to regard it as secondary. In other animals it may 

 remain indefinite long after fertilization, or as I believe, long after the 

 gastrula stage. In salpa it is fixed by external limitations, but the 

 observations of Seeliger (9), Davidoff (16), and Van Beneden and Julin, 

 all show that the egg of clavelina does not manifest it until the four- 

 celled stage is reached, and Metschnickoff says that in another ascidian, 

 Ascidia mentula, no trace of it is exhibited until after the numerous cells 

 of the blastula have become differentiated into large vegetative and 

 small animal cells. I think that it is plain that if its early appearance is 

 the result of secondary acceleration, we cannot deduce from the fact of 

 its early appearance in the embryos of the bilateral, coelomatous metazoa 

 any conclusion whatever regarding their common ancestry. 



