Yolk of Tehostean Ova. 85 



these spheres that the species to which an ovum belongs may 

 often be determined at a glance by their aid. 



Struck by their diagnostic significance, Agassiz and 

 "Whitman divided pelagic eggs into two great divisions, 

 those which are provided with one or more oil-globules and 

 those which are not so distinguished *. Such a division has, 

 however, little value, as the occurrence of these large globules 

 is apj)arently most erratic — the ova of closely-allied species 

 exhibiting the utmost diversity in this respect. Indeed the 

 American observers themselves describe an ovum of a Pleuro- 

 nectid [PseudorJiomhus ohlongns) showing a large oily sphere, 

 a structure not present in the egg of any other species of 

 flounder known to zoology. The ova of the Gadoids, too, 

 are marked by the absence of such structures ; yet a remark- 

 able exception has been recently found at St. Andrews, viz. 

 the hitherto undescribed pelagic egg of the ling [Molva vul- 

 garis) y which exhibits a single oleaginous sphere of a pale 

 green tint. Additional exceptions are furnished by the 

 closely-allied freshwater species. Lota vulgaris (the oil-globule 

 in which, as described by Van Bambeke f, is almost precisely 

 like that in the e,gg of the ling), as well as the eggs of the 

 American Brosmius % and Motella mustela §, while the ova of 

 all other Gadoids at present known lack this marked feature, 

 no large globule being present in the case of the cod, had- 

 dock, whiting, bib, &c. Again, we find amongst freshwater 

 forms that the ova of the salmon, trout, and grayling have 

 large rufous-tinted spheres enveloped in the deutoplasm or 

 yolk, whereas in other forms which inhabit the same waters 

 and deposit their ova in similar situations, such as the pike ||, 

 tench, roach ^, &c., no such globules are present. A classifi- 

 cation of ova founded on the presence or absence of these 

 spheres presents a strange medley — the two lists bringing 

 together widely separated species and placing side by side 

 fishes with pelagic and demersal ova, and most diverse fresh- 

 water and marine forms. An undoubted specific value belongs 

 to these spheres ; but no generic or wider diagnostic signifi- 

 cance can be attributed to them. Certainly the interpreta- 

 tions which the presence of these globules has hitherto 

 received are not only unsatisfactory, they are undoubtedly 



* ' Studies from Newport Marine Laboratory. — XVI. Dev. Oss. Fishes,' 

 p. 2 (1885). 



t " Eecherclies sur rembryologie des poissous osseux,'' M6m. Cour. de 

 I'Acad. Roy. de Belgique, tome xl. p. 5. 



X U. S. Fish. Comm. Rep. 1882, p. 407. 



§ G. Brook, Journ. Linn. Soc, Zool. vol. xviii. p. 208. 



II E. B. Truman, Month. Microsc. Journ. vol, ii. 1869, p. 188. 



^ Van Bambeke, loc. cit. pp. 2 and 13. 



