32 EEPRODUCTIOX AND DEVELOPMENT OF TELEOSTEAN FISHES 



homogeneous and colourless ; at its surface is a large oil-globule, 

 also colourless, having a diameter of '32 to "33 mm. The blastodisc, 

 and the blastoderm at the commencement of segmentation, has a 

 slightly yellow colour, which disappears later. The envelope is 

 thin, and I have not noticed any sculpturing or inequalities on its 

 outer surface. The perivitelline space is small, at the stage referred 

 to, consisting as usual of only a ring-shaped cavity in the depression 

 between the blastoderm and the yolk ; the rest of the ovum is in 

 contact with the inner surface of the envelope. 



The oil-globule moves with perfect freedom at the surface of the 

 yolk, which proves that the latter substance is a liquid of very slight 

 tenacity. When the ovum is floating freely in water the blastoderm 

 is at the lowest pole, and the yolk at the uppermost. When the 

 ovum is placed upon a slide, whatever be the position of the blasto- 

 derm, the oil-globule rises to the uppermost pole ; the blastoderm is 

 the heaviest portion of the ovum, the oil-globule the lightest. On 

 the sKde the position of the blastoderm is proved to be fixed in 

 relation to the position of the egg envelope, but the oil-globule 

 remains free and rises to the pole which is vertically highest under 

 the action of gravity. Even when the blastoderm is placed upper- 

 most the oil-globule passes freely beneath it and rests below its 

 centre. Thus it is evident that the yolk is to be regarded as a 

 liquid enclosed within a layer of protoplasm continuous with the 

 blastoderm, and at the surface of this liquid next to the protoplasmic 

 layer moves the oil-globule. 



The blastoderm spreads out, and the segmentation cavity, 

 embryonic rudiment, and embryonic ring are formed in the usual 

 way. At the temperature of about 13*5° C. the sixteen-cell stage is 

 reached in about seven hours, and the segmentation cavity is 

 formed before the end of the first day (twenty-four hours). On the 

 second day the growth of the blastoderm over the yolk takes place. 

 During all this time until the envelopment of the yolk is completed 

 the oil-globule remains movable (figs. 17 and 18), but as soon as 

 the yolk is completely covered by the blastoderm the oil-globule 

 becomes fixed in a position ventral to the posterior end of the 

 embryo (figs. 19, 22, 24). This fixture is effected by the periblast, 

 and is a fact which deserves the greatest attention. The oil-globule 

 seems to belong entirely to the yolk, and the periblast grows with 

 the blastoderm outside the yolk. Why then should it grow in 

 between the oil-globule and the yolk, separating the two and fixing 

 the former in one position ? In its new condition the oil- globule 

 projects slightly beyond the general surface of the yolk, which is 

 depressed in its immediate neighbourhood. Thus there is a cavity 

 round the projecting part of the oil-globule. This projecting part 



