of the Fishery Board for Scotland. 



281 



The ducts, which at first were more or less ventral, seem to lie on the 

 same level as the ventral part of the notochord, but a little distance from 

 it. As the ducts are developed anteriorly, they do not approach each 

 other, but possibly the distance between them is increased. The ducts 

 become coiled at their anterior ends, so that transverse sections show not 

 a pair, but as many as three pairs of ducts in the same plane (figs. 27 

 and 28, prn. The duct at its anterior extremity is bent on itself, and the 

 inner limb is continued posteriorly till it opens into the capsule lodging 

 the glomerulus. This opening is seen on the left side in the sections (figs. 

 27 and 28). The pronephric tubule on the eighth day, as noticed, com- 

 municates with the body cavity, but when the glomerulus is developed in 

 embryos of about twenty days, the tubule is no longer in communication 

 with the coelom, but opens into the closed glomerular cavity. The 

 glomerulus is formed immediately below the aorta, and doubtless arises 

 from the loose trabecular tissue under the notochord from which the aorta 

 is formed. In some sections the glomerulus appears as a single median 

 structure, but four or five days after hatching the structure is paired. 

 Even in older forms the division of the two glomeruli is still incomplete. 

 In fig. 28, gl., which represents a section of an embryo nine days hatched, 

 the division is not so manifest as in fig. 27, gl., which is younger by four 

 days than that of fig. 28. The glomerulus exhibits a vascular structure, 

 and the tissue between the pair forms the mesenteric connection of the 

 intestinal tube. They are situated in the same region as the gall bladder, 

 and the pronephric tubule is continued forward to about the anterior end 

 of the liver. 



The segmental duct is undoubtedly derived from mesoblast in the 

 plaice. Epiblastic invaginations are present in figs. 19 and 20, l.s.o., 

 which might be mistaken for renal thickenings, but their position and 

 their limited extent mark them out as lateral sense organs. These 

 apparently correspond to ' the thickening of that portion of the surface 

 epiblast, overlying the intermediate cell-mass,' which Brook * described 

 in the trout as renal. Whatever may be the case in the development of 

 the segmental duct in Mammals, Amphibians and Elasmobranchs, my 

 results coincide with these of Rosenberg, f Oellacher,J and Hoffman § on 

 the teleostean kidney, as to the origin of the segmental duct. 



As to the opening of the tubule, there is a single opening for either side 

 in the early stages to the body cavity, and later to the glomerular cavity, 

 but there is no trace of multiple openings, as in Amphibians and Selachians, 

 and in the segmental organs of Annelids. 



The Pleuronectidas of our seas lack a swim-bladder. Having regard to the 

 fact that the Physoclistous Teleosteans, while they have a ductus pneumati- 

 cus during larval life like the Physostomi, lose this as development advances, 

 and that no trace of a swim-bladder had been obtained in the Pleuronec- 

 tidse, I examined the young stages particularly of the plaice, in order to 

 ascertain if any rudiment of the swim-bladder was present. In an embryo 

 of the eighth day, that is at the same time as the appearance of the heart, 

 I found that a small furrow was formed along the dorsal wall of the diges- 

 tive tube, and that it was continued through a dozen sections. Figs. 16 

 and 17 show this furrow in the middle and at one end. This furrow soon 

 disappears, and it is the only dorsal diverticulum which can correspond 

 with the swim-bladder diverticulum of other Teleosteans. It appears one 



* 1 Note on the Epiblastic Origin of the Segmental Duct in Teleostean Fishes and 

 in Birds,' Proc. Royal Society, Edin., 1887. 



+ ' Untersuchungen iiber die Entwick, der Teleosteierniere.' Inaug. Dissert, Dorpat 

 1867. 



% Loc. cit. 



§ Loc. cit. 



