OCCURRING IN THE NEIGHBOURHOOD OF PLYMOUTH. 11 



i. e. the pole opposite to the centre of the blastodisc. But the oil- 

 globule, as is usual in similar ova, is able to move freely at the 

 surface of the vitellus at the early stages of development, that is, 

 until the vitellus is completely enveloped by the blastoderm, and 

 therefore when the ovum is placed on a slide in any position the 

 oil-globule usually rises to the upper pole. 



The ovum shown in fig. 1, at the stagfe when the blastoderm had 

 just begun to spread, measured *97 by '98 mm. That shown in 

 fig. 2, when the embryo was fully formed but the tail had not 

 begun to grow out, measured 1'2 mm. in the shortest diameter, 

 lb mm. in the longest. The oil-globule is "19 mm. in diameter. 

 The ova were measured lying on a slide in sea-water without a 

 cover-glass, the measurements being made by first tracing the 

 outline of the ovum with Zeiss's camera, and then throwing the 

 image of a millimetre scale on the tracing. In the later stage the 

 increase in size is due to an expansion of the egg-envelope, the peri- 

 vitelline space having increased. But ova of a given species are 

 always, within narrow limits, variable in size. 



The development was not carried to a later stage than that shown 

 in fig. 2. At that stage black chromatophores had appeared at the 

 sides of the embryo near the dorsal median line, as black specks. 



Tkigla cuculus. 



I obtained some ova of this species on April 5th and Gth when on 

 board a trawler south of the Wolf Rock, and also some milt, and 

 thought I had effected artificial fertilization, but when I examined 

 the ova on shore I found the fertilization had not succeeded. 

 Another sample, this time successfully fertilized, was brought in by 

 the Laboratory fisherman on April 28th ; they were taken, together 

 with eggs of the common sole and merry sole [Pleuronectes mkro- 

 cephalus), on April 27th, about forty miles north of the Longships 

 Lighthouse. On May 13th the Laboratory fisherman returned from 

 a trip on a trawler to the neighbourhood of the Wolf Rock, bringing 

 one bottle of ova of this species fertilized on May 10th, and a bottle 

 of ova of Trigla gurnardus fertilized on May 11th. A few more ova 

 of Trigla cuculus I fertilized myself at the same locality on May 16th. 

 The natural conditions to which these ova are exposed during deve- 

 lopment are thus the same as in the case of those of Pleuronectes 

 microcephalus (see below), the period of spawning being the same 

 for both species. Trigla gurnardus spawns also at the same time. 



With regard to the experimental conditions I had not time to pay 

 so much attention to this as to other species. My observations are 



