ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 7C7 



In sca-water tadpoles soon die, but in a dilute solution (0 • 2 to • 8 

 per cent.) of the salts of sea-water development will take place, 

 though very much more slowly than in fresh water. By the produc- 

 tion of artificial waves, development can be procured even in more 

 concentrated solutions. 



Development of Food-Fishes.* — Mr. E. E. Prince has studied, 

 and reared from the ova, embryos of Gadus merlangus, G. ceglefinus, 

 G. morrhua, Trigla gurnardus, Pleuronectes flesus, and P. limanda. 



"With the exception of the herring, the ova of the principal food- 

 fishes are pelagic, and when mature are almost identical in appear- 

 ance and structure. The young ovarian ovum is more or less opaque, 

 but becomes more transparent as it approaches maturity ; it then 

 exhibits the following structures :— (a) A homogeneous yolk, desti- 

 tute of large globules, except in T. gurnardus ; (b) a delicate cortical 

 film of protoplasm ; (c) a space or breathing chamber, separating the 

 vitellus from (d) the external capsule or yolk-sac. This capsule is 

 hyaline, tough, destitute of pores or striatums, although thin and 

 transparent ; its thickness varies in different species ; it is pierced 

 by one aperture, having an hourglass shape. These pelagic ova 

 show no tendency to adhere together, but float freely and separately ; 

 in still water they may congregate in masses. If no spermatozoa 

 come near them, they become opaque and sink ; the entrance of 

 spermatozoa has never been seen, but one is probably sufficient, and 

 enters the micropyle. 



The "blastodisc" is formed on the pole of the egg which is 

 carried downwards, contrary, therefore, to what happens in am- 

 phibian ova ; the vitellus revolves freely in the " yolk-sac " so that the 

 blastodisc can always regain its ventral position when disturbed. 

 Segmentation goes on in such a way that more or less irregular cells 

 are produced ; this is not confined to the blastodisc, but new cells 

 are formed apparently in the periblast, which then become added to 

 the disc. The exact mode of development of the blastodermic rim is 

 uncertain ; a thickening of this rim is produced corresponding to a 

 radius of the blastodisc ; this thickening is the rudiment of the 

 embryo. In the formation of the nerve-cord, no trace is found of a 

 medullary groove or growing-in of the corneous layer as described by 

 Calberla, but a fissure appears later on by dehiscence. The noto- 

 chord has been formed from hypoblast, and now becomes detached ; 

 posteriorly it is continuous with an undifferentiated mass of cells. 

 Close on each side of the notochord are the mesoblast cells. The 

 cephalic end of the embryo is fixed, but the caudal end advances with 

 the blastoderm. The tail lies sideways on the yolk, and continues in 

 this state of torsion till the embryo is free. 



Nothing particular is noticed about the development of sense- 

 organs, which, as is well-known, appear as solid instead of as hollow, 

 outgrowths. The notochord is surrounded by a cuticular sheath, 

 secreted by the outermost cells ; round this a mesoblastic sheath is 



* Ann. and Mag. Nat. Hist., xvii. (1S80) pp. 443-61. 



