698 KEPOET— 1886. 



also derives the pectoral girdle from a branchial arch, and, according to both 

 authorities, this arch must have shifted to the surface from a deeper plane of 

 origin, for the girdle is essentially a superficially placed structure. The visceral 

 cleft system arises more deeply, and is hypohlastic, the gill-arches being developed 

 in the splanchnopleure, whereas the cartilaginous pectoral girdle is somatopleuric, 

 if the cells, from which it originates, are to be distinguished from the adjacent 

 intermediate-cell mass. There are many difficulties in deriving the fin from 

 branchial arch appendages, even so primitive a girdle as that of Ceratodus having 

 no directly articulating rays, comparable to branchiostegal elements, and there are 

 difficulties no less in assigning a branchial origin to the shoulder-girdle itself. The 

 girdle arises as two separate rods, external to the heart and the alimentary tube, 

 and passing dorso-ventrally. Its two elements originate, indeed, precisely like a 

 pair of strongly developed ribs, for the ribs begin as ossifications at independent 

 centres in the intermuscular septa, and grow both ways, dorsally to unite with the 

 vertebral bodies, and ventrally to meet or remain separate, as the case may be. In 

 exactly the same way the scapular and coracoidal halves of the pectoral bars 

 develop to subserve the same function — viz., that of providing a support in connec- 

 tion with the movements of the body. 



The attachment of the pectoral girdle to the skull recalls the connection of un- 

 doubted rib-elements with the skull in Carps and Siluroids. Can we not refer the 

 two halves of the pectoral girdle to the axial skeleton as strengthened and modified 

 costal rods,' possibly equivalent to many coalesced ribs, to which the fins, originating 

 independently, become secondarily attached ? It is significant that the Cyclostomes 

 are destitute of ribs, and they possess no limbs ; and this accords with the above 

 suggestion, for if no ribs exist, no limb-girdle could be developed from them. In 

 the Selachians it is noteworthy that the ribs seem to have sufiered great degenera- 

 tion. Whether a costal or branchial origin be attributed to the pectoral girdle, the 

 appended limbs are wholly separate structures, and only become related to it after 

 changes in shape, position, and function of the most remarkable character. 



12. Some Remarks on the Egg-Memhranes of Osseous Fishes. 

 By Robert Scharff, Ph.D., B.Sc. 



It is no doubt due to the transparence and the extreme smallness of the objects 

 that so many difierent opinions prevail on the structure of the Teleostean ovum. 

 All authors, however, agree that a membrane surrounds the egg, which in many 

 if not in all cases, is pierced \>j minute canals or pores. The name most generally 

 adopted by zoologists for this membrane is ' zonaradiata,' which I think is a much 

 more suitable one than ' vitelline membrane,' or ' yolk sac,' for reasons which I shall 

 give presently. 



In the ovum of the gurnard {Trigla gurnardus) I could distinctly see with a 

 high power a very delicate homogeneous membrane internally to the ' zona radiata.' 

 In the ripe egg, or rather in the fully-grown intra-ovarian egg, it covers the proto- 

 plasmic layer known as the ' Eindenschicht,' or * periblast,' which is one of the 

 later modifications of the yolk. Ransom was the first observer who not only saw 

 a similar membrane in various Teleostean eggs, but also isolated it. Some of the 

 later observers failed to make it out ; others, again, confirm Ransom's observations. 

 As long as any doubts exist as to presence or absence of this inner ' yolk sac ' of 

 Ransom, the ' zona radiata ' should not be called a vitelline membrane. The latter 

 term, however, might veiy well be applied to the inner membrane which I have 

 just mentioned. It corresponds to a cell-membrane, and is, therefore to be con- 

 sidered as a vitelline membrane. 



In the gurnard as well as in the cat-fish {Anarrhichas lupus) the zona radiata 

 showed an external portion which stained darker, and through which the pores 

 piercing the inner part were apparently continued. This outer part was sometimes 



' Prof. Humphry, of Cambridge, hinted at a connection of certain elements in 

 the coalesced lateral fins (that is to saj, the median longitudinal fins) and the ribs, 

 in the Journ. of Anat. and Plnjs. vol. s. p. 671. 



