INTRODUCTORY. 29 



stages of development while floating at the surface, in 

 the same manner as had been previously ascertained in 

 the case of the cod. M. Sars writes to us that he was 

 at first inclined to believe this development of the ova 

 while floating was peculiar to the members of the 

 Gadidce or cod famil}^ in its restricted sense ; but in 

 the summer of 1865 he visited the southern coast of 

 Norway during the season for mackerel, and found 

 abundant evidence of the same rule obtaining in the 

 case of that widely distinct fish. There is, we believe, 

 good reason for thinking that the actual sj^awning of 

 the mackerel (Scomber) takes place at the surface, and 

 that the ova do not rise from below, as may be the 

 case with the spawn of fish which, like the cod and 

 haddock, usually keep near the bottom ; but in saying 

 this we are only judging by the ordinary habits of the 

 two last-mentioned species, and we have no special 

 ground for assuming that those habits do not alter at 

 the spawning season. It is not a very important ques- 

 tion, however, for if the specific gravity of the ova be 

 such as will keep them at the surface, it must be quite 

 capable of bringing them there from any depth at 

 which they may have been produced. Besides the ova 

 of the mackerel, which were obtained in all stages and 

 successfully hatched out — the young mackerel being 

 easily recognized — the ova of at least four other kinds 

 of fish were procured ; " the ova being readily distin- 

 guished from each other both by size and by the colour 

 and form of the embryo." M. Sars adds — " In some of 

 them there was, as in the ova of the mackerel, a very 

 discernible pellucid oil-globule at the upper pole, the 

 magnitude and relative position of which to the 

 embryo were characteristic of each sort. Of most of 

 them I was able to get the young excluded, but they 



