STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 8ll 



the dorsal aspect of the body-cavity. It is covered ^>n- its 

 ventral aspect by the oviduct and by its own duct, but is sepa- 

 rated from both of these by a layer of the tough peritoneal 

 membrane, through which the collecting tubes pass. It extends 

 forwards from the anus for about three-fifths of the length of 

 the body-cavity, and in our example had a total length of about 

 28 centims. (Plate 39, fig. 60, k}. Anteriorly the two kidneys 

 are separated by a short interval in the median line, but poste- 

 riorly they come into contact, and are so intimately united as 

 almost to constitute a single gland. 



A superficial examination might lead to the supposition that 

 the kidney extended forwards for the whole length of the body- 

 cavity up to the region of the branchial arches, and Hyrtl appears 

 to have fallen into this error ; but what appears to be its anterior 

 continuation is really a form of lymphatic tissue, something like 

 that of the spleen, filled with numerous cells. This matter 

 (Plate 39, fig. 60, ly.) continues from the kidney forwards with- 

 out any break, and has a colour so similar to that of the kidney 

 as to be hardly distinguishable from it with the naked eye. The 

 true anterior end of the kidney is placed about 3 centims. in 

 front on the left side, and on the same level on the right side 

 as the wide anterior end of the generative duct (Plate 39, fig. 

 60, od.}. It is not obviously divided into segments, and is richly 

 supplied with malpighian bodies. 



It is clear from the above description that there is no trace of 

 head-kidney or pronephros visible in the adult. To this subject 

 we shall, however, again return. 



As will appear from the embryological section, the ducts 

 of the kidneys are probably simply the archinephric ducts, but 

 to avoid the use of terms involving a theory, we propose in the 

 anatomical part of our work to call them kidney ducts. They 

 are thin-walled widish tubes coextensive with the kidneys. If 

 cut open there may be seen on their inner aspect the numerous 

 openings of the collecting tubes of the kidneys. They are 

 placed ventrally to and on the outer border of the kidneys 

 (Plate 39, fig. 60, s.g.\ Posteriorly they gradually enlarge, and 

 approaching each other in the median line, coalesce, forming 

 an unpaired vesicle or bladder (bl.} about 6 centims. long in 

 our example opening by a median pore on a more or less 



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