OCCURRING IN THE NEIGHBOURHOOD OP PLYMOUTH. 21 



The layer of separate vitelline segments is not exclusively con- 

 fined to the ova of Solea ; it is the combination of this character 

 with the peculiar arrangement of the oil-globules which distin- 

 guishes this genus. The external segmental layer of the yolk was 

 first noticed by Agassiz and Whitman* in a species of ovum which 

 they ascribe to Temnodon saltator, Linn., known in America as the 

 blue-fish. Temnodon belongs to the same family as the boar-fish, 

 Capros aper, whose ova have been described above, namely, the 

 Carangidse. Agassiz and Whitman state tbat when the blastoderm 

 has enveloped the yolk the yolk-segments are absent immediately 

 beneath the embryo, but I have not verified this in Solea. In the 

 ova described by these authors there is but a single oil-globule of 

 considerable size, and judging from the figures which show the 

 globule in different positions it is mobile at the early stages. 



Raffaele describes a perfectly similar superficial layer of seg- 

 mented yolk in Mullus surmuletus, L., the red mullet. He informs 

 us, moreover, that in the ovarian ovum of Mullus when it is 

 approaching maturity the yolk-segments are in the centre of the 

 ovum, and are nothing but a portion of the vitelline segments which 

 at an earlier stage make up the whole mass of the yolk. Most of 

 these segments fuse together to make up the homogeneous part of 

 the yolk ; the remainder pass to the surface and take up a position 

 beneatli the germ or blastodisc, persisting during development. 

 The partitions enclosing the segments Raffaele believes to be proto- 

 plasmic and continuous with the protoplasm of the germ, by which 

 he explains the fact that the yolk-segments are involved in the 

 movement of the blastoderm. The explanation is in all probability 

 correct. 



The superficial layer of segments is also described by Raffaele in 

 the ovum of Callionymus festivals, and in an unidentified ovum 

 (Species No. 2 in his paper) with a diameter of '75 mm. which he 

 says resembles that of Callionymus. He states that in G. festivus 

 the layer of segmented yolk extends all round the ovum from the 

 beginning, even in the mature ovum before fertilization, and under- 

 goes no movement during the extension of the blastoderm. He 

 refers to a description by Mcintosh of the ovum of G. lyra (Ann. 

 Mag. Nat. Hist., vol. xvi, 1885), in which it is stated that the sur- 

 face of the vitelline membrane in this species exhibits a hexagonal 

 mosaic of raised lines, and says that nothing of the kind being 

 visible in the ovum of C. festivus, Mcintosh probably saw the folli- 

 cular epithelium attached to the ovum when it was taken from the 

 ovary, and mistook this for a marking of the vitelline membrane. 



* Pelagic Stages of Young Fishes, Memoirs of Museum of Comp. Zoology, vol. xiv. 

 No. 1, pt. i, Cambridge, 1885. 



