EGGS AND LARVAE OF TELEOSTEANS. 99 



outer surface of the external layer or zona externa is adhesive, and the 

 ruptured edge becomes attached, so that the ovum swings in the water from 

 the flexible suspensory membrane thus formed. I have elsewhere * described 

 the separation of the two layers of the zona resulting in the formation of the 

 suspensory membrane, but the relation of the united parts of the two layers to 

 the micropyle is now described for the first time, and is shown in fig. 6 as it 

 appears in optical section. 



When fertilisation takes place, a large perivitelline space is formed by the 

 elevation of the internal zona. Unfortunately, I was unable to obtain a sufficient 

 number of healthy ova to study the development Fig. 5 was taken from an 

 ovum fertilised at Stirling on May 6 ; it was drawn on May 7, twenty-five 

 hours after the egg was shed. It shows the character of the egg, and the rela- 

 tion of the blastodisc to the yolk ; but the blastodisc was not segmented, and 

 it is possible that the ovum was not really fertilised, the formation of the 

 blastodisc occurring normally in ripe Teleostean ova without fertilisation. The 

 ovum resembles somewhat that of the herring. It is a little more transparent 

 than the herring ovum, and the structure of the yolk is different. In the egg 

 of Osmerus there are a number of oil globules, varying much in size, while the 

 yolk of the herring ovum has no oil globules. The diameter of the fertilised 

 ovum is 1*3 mm. 



4. Pleuronectes platessa, Linn. (Plaice) (PL II. figs. 1-3). 



The eggs of the plaice were artificially fertilised on board a steam trawler 

 outside the Isle of May, February 3, 1886. The egg is 1*95 mm. in diameter, 

 and like the other eggs of the Pleuronectidse which I have examined, has a 

 perfectly homogeneous yolk. The perivitelline space is small. The larvae were 

 not actually hatched, but one taken from the ovum, when almost ready to hatch, 

 is shown in fig. 3. Its length is 4*1 mm. The eye is faintly pigmented. There 

 are three rows of yellow dendritic pigment cells down each side, and black 

 dendritic cells in the head. The anus is open, and situated immediately behind 

 the yolk sac. The notochord is multicolumnar ; the pelvic fin not developed. 



5. Pleuronectes fiesus, Linn. (Common Flounder) (PI. II. figs. 4-8). 



Eggs of this species were obtained on March 30, 1886, in the Firth of 

 Forth, in Aberlady Bay. It is the only species which has been found in 

 abundance, and in the spawning condition, so far up the Firth. The egg is 

 similar in all respects to that of the plaice except in size. It is 1 -03 mm. in 

 diameter. The newly hatched larva is transparent, and 3*01 mm. in length. 



* Proc. Zool. Soc, London, 1886. 



