chap, xix.] LATERAL HOMOEOSIS. 465 



II. Homoeosis in cases of normal Bilateral Asymmetry. 



In proportion as an animal is bilaterally symmetrical the right 

 side is an image of the left. Nevertheless in many substantially 

 symmetrical forms there is asymmetry in the condition of some 

 one or more organs present on both sides. (This asymmetry, in 

 the cases to be considered, is of course distinct from that due to 

 asymmetrical disposition of unpaired viscera, such as the heart and 

 liver of vertebrates, &c.) In several of these cases there is evidence 

 that both sides may on occasion assume the form normally proper 

 to one only. 



Some one will no doubt be prepared with the suggestion that 

 these variations are reversions : with this suggestion I shall deal 

 after the facts have been recited. 



Spiracle of Tadpole. 



721. Pelobates fuse us : a tadpole, 7 cm. long, having two spiracles 

 symmetrically placed (Fig. 151), one on the right side and the other on 

 the left 1 . [No details given.] H^ron-Royer, Bull. soc. zool. Fr. t ix. 



Fig. 151. A tadpole of Pelobates fuscus, having, as a variation, two spiracular 

 openings, No. 721. (After Heron-Royek.) 



1884, p. 162, j%. [In the normal there is only one spiracle, that of 

 the left side. In Pipa and Dactylethra two spiracles are normally 

 present. See Wyman, Proc. Bost. N. H. S., ix. p. 155; Wilder, Am. 

 Nat.y 1877, xi. p. 491 ; Boulenger, Bull. soc. zool. Fr., 1881, vi. p. 27. 



Tusk of Narwhal. 



722. Monodon monoceros (Narwhal). In normal males the left tusk 

 alone is developed while the right remains abortive in its alveolus. In 

 the female both tusks are in this rudimentary condition. No reliable 

 record (1871) of a specimen having the right tusk only developed, but 

 in eleven cases from various sources the two tusks were both developed, 

 and in several of these the two were of about equal length. The 

 normal asymmetry of the skull is not affected by the presence or 

 absence of the teeth. Clark, J. W., P. Z. S., 1871, p. 42, figs, (full 

 literature); see also Turner, W., Jour. Anat. Phys., 1871, p. 133 and 

 1874, p. 516. 



Ovary and oviduct of Fowl. 



It might be anticipated that development of the right ovary and 

 oviduct in birds would be a frequent form of Variation, but as a 

 matter of fact very few such cases are recorded. In consideration of 



1 In the same place is recorded a case of a tadpole of this species having the 

 spiracle on the right side instead of the left, perhaps a case of situs inversus. 



b. 30 



