CTENOLABEUS ADSPEESTJS. 19 



ture which could serve as a reliable means of identifying the two kinds of 

 eggs prior to the time of hatching. The eggs of the Gunner are, on the 

 average, a few hundredths of a millimeter larger than those of the Flounder 

 (P. americanus), but this distinction is full)- bridged over by variations on 

 both sides. Although the periods of occurrence are consecutive, they overlap 

 each other, and hence do not afford even an indirect means of identification. 

 The character of the pigment, its time of appearance, and its development 

 appear to be the same in both kinds of eggs. At the time of hatching there 

 is a very constant difference in the size and shape of the yolk-sac. that of 

 Ctenolabrus being comparatively large and ovate in form, while that of the 

 Flounder is proportionally smaller, and oblong-elliptical in shape. The 

 Flounder is longer and more slender at the time it leaves the egg-membrane 

 than the dinner of a corresponding age. 



For embryological study, the eggs of the Gunner offer special advantages 

 in the way of easy and abundant supply of material; for they are obtainable 

 at all hours of tin- day, either directly from the sea or by artificial fertilization. 

 Most of our material for the study of the early stages has been derived from 

 this source; and as the teleostean type of development is well exemplified in 

 these eggs, we have given in Plates VII. to IX. a selection of the principal 

 stages, forming an outline sketch up to the time of hatching, reserving a 

 more detailed account for the second and third parts of this memoir. 



The condition of the freshly laid egg, which measures from .85 to .92 mm. 

 in diameter, is shown in Plate VII. fig. 1. The whole cortical layer of 

 the egg, which is now everywhere of the same thickness, is densely filled 

 with refractive granules, which render the egg quite opaque. A few mo- 

 ments after fecundation all or nearly all of these granules have dissolved. 

 leaving the egg completely transparent. Immediately after the penetration 

 of the spermatozoon a disc-like thickening of the cortical layer appears at 

 the lower pole of the egg; and at the centre of this disc, may be seen, in 

 mounted preparations the minute male pronucleus. It is a curious fact, of 

 which the proof will be given in our second memoir, that the male pronu- 

 cleus becomes the centre of attraction around which the discoidal aggregation 

 of protoplasm takes place, and towards which, after the formation of (lie sec- 

 ond polar globule, the female pronucleus gravitates. In the course of a few 

 minutes the blastodisc becomes quite prominent, as shown in Figs. •_' and 3; 

 and a few minutes later the first cleavage-furrow appears, dividing the 

 disc into two nearly equal parts (Fig. 4). A second furrow at right angles 



