2 THE PELAGIC STAGES OF YOUNG FISHES. 



The first part of the present memoir is devoted to descriptive sketches 

 of the different fish eggs which have come to our notice. We have, as far 

 as practicable, given the most characteristic stages of the egg of each species, 

 and have, whenever possible, raised the young until we could recognize in 

 the embryo the young stage of some fish already known to us from our 

 pelagic fishing. Many of the sketches here given of young fishes supple- 

 ment those formerly published by Mr. Agassiz, and we have added a synop- 

 tic table of the various eggs and young fishes we have thus far observed, 

 with references to the plates where they are figured. The difficulties of 

 connecting the eggs and the free-swimming embryos is very great, and 

 undoubtedly it will be found in the future, as we have already observed, 

 that many of our identifications are not correct. The differences which dis- 

 tinguish the eggs of widely separated species are often very slight, and Ave 

 found also that the variation in the size and ornamentation of the embryo 

 was frequently very considerable. In fact, individual differences are early 

 developed, and the peculiarities of the full-grown fish are hinted at most 

 plainly either in the egg or in the very youngest embryonic stages after 

 hatching. 



A comparison of successive stages of growth (but not of the same indi- 

 viduals) of such young fishes as we figure in Plates III., XIII., XIV., XV., 

 XVI., and XIX. will illustrate this point more fully than detailed descriptions 

 of the different outlines and patterns of coloration of the successive stages. 



We have also for one species (Ctenolabrus) illustrated quite at length 

 (Pis. VII., IX.) the different stages of segmentation and the general appear- 

 ance of the embryo within the egg before hatching. 



The second part of this memoir will be devoted to the earlier stages of 

 cleavage and the formation of the embryonic ring; after this we shall take 

 up as fast as practicable the formation of the embryo, the fins, the chroinato- 

 phores, the nervous system, and the alimentary canal. 



The pelagic eggs we have had occasion to examine divide themselves 

 into two great divisions, — those which are provided with one or more oil- 

 globules, and those which are not. This distinction at first sight appears a 

 most important one; yet if we examine eggs without globules, we shall find 

 that the yolk-mass is thoroughly permeated with minute fatty globules. 

 But these minute globules never coalesce, and do not form a number of 

 small globules, as is the case in some of the Cottoids (PL I. figs. 1-3). We 

 may indeed consider these eggs with many globules as intermediate between 



