59 



The branchiostegal rays support a membrane which is externally for the greater part 

 of its length continuous with the opercular flap, but is free at its edge, and extends 

 somewhat beyond the flap ; it forms a concavity which directs the water pressed out 

 in expiration upwards towards the base of the pectoral fin. The edge of the branchi- 

 ostegal membrane is, in ordinary respiration, kept pressed against the posterior wall 

 of the branchial cavity except opposite the base of the pectoral fin, where the membrane 

 forms a small aperture by which the water that has passed over the gills escapes. The 

 opercular flap supported by the opercular bones forms the external wall of the 

 branchial chamber, in which the branchiae are contained and concealed, but the oper- 

 cular flap is morphologically nothing but a fold of skin projecting backwards from the 

 hyoid arch and supported by dermal bones. The branchiostegal rays represent the 

 branchial skeletal rods of the lower part of the hyoid arch : they have been much 

 enlarged, and the filaments they supported have lost their branchial function, and 

 coalesced to form the branchiostegal membrane. 



Minute Structure of the Reproductive Organs and Development of the Reproductive 



Elements. 



It has already been mentioned that the ovary consists of a hollow tube from the inner 

 surface of the walls of which project a number of longitudinal lamellae containing the 

 ova. A section of a young ovary of the right side is represented in PI. XIII, 1, as 

 seen under a low magnifying power, namely Zeiss' objective A, ocular 2, The specimen 

 from which this ovary was taken was 7^ in. long, and was immature. But the 

 structure of a mature ovary is similar in all respects except the size of the ova. 

 The wall of the ovary consists of laminated fibrous tissue, and the ovigerous lamellae 

 are supported by projections of this tissue into the interior of the ovary. On the 

 dorsal side of the transverse section are seen two blood vessels, the ovarian artery and 

 vein. The ovigerous lamellae are absent from, a small portion of the ventral wall of 

 the ovary, where the internal surface of the wall is smooth and barren. The surface 

 of the ovigerous lamellae is covered by a thin epithelium of cells too minute to be 

 distinguished separately under a low power. Here and there in this epithelium young 

 ova are seen developing. The epithelium is called the germinal epithelium, and 

 from its cells all the ova are originally derived. The older ova are seen below the 

 epithelium in the substance of the ovigerous lamellae. The lamellae contain in their 

 centre a plate of somewhat dense fibrous tissue, from which a loose reticulum of 

 fibrous strands and bands extends to the germinal epithelium. The older ova sink 

 into this loose fibrous layer, and each ovum is surrounded and enveloped by strands 

 of the fibrous tissue. The spaces in the fibrous reticulum are doubtless in life filled 

 with lymph, while the tissue itself is richly supplied with capillary blood vessels. The 



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