286 LECTURE XII. 



LECTURE XII. 



GENERATION AND DEVELOPMENT OF FISHES. 

 ' MALE ORGANS. 



All fishes are dioecious or of distinct sex. The male parts of 

 generation present a progressive gradation of complexity from 

 the essential gland, or testis, as a single organ distinguishable 

 only by microscopic examination of its contents from an ovarium, 

 to a more definite and concentrated form of testis with complete 

 bipartition, then to the development of a proper duct or ' vas de- 

 ferens,' next of a vesicula seminalis and prostate, aftervs^ards of an 

 intromittent organ, and finally of superadded 'claspers,' or mecha- 

 nical instruments for retention of the female iii coitu. The pre- 

 paration of the Petromyzon marinus (No. 2373.) shows the testis 

 in the form of a series of thin transverse lobes, or folds closely 

 attached by a duplicature of the peritoneum to the median line 

 of the back of the abdomen, between the kidneys ; the extension 

 of the over-lapping oblique folds to the right and left of the line 

 of attachment indicates the duplex character of the gland. Its tissue 

 consists of small spherical cells filled Avith the minute corpuscular 

 spermatozoa.* These escape by dehiscence of the cells and rupture 

 of the peritoneal covering into the abdominal cavity, and are expelled 

 by reciprocal pressure of the intertwined sexes, from the peritoneal 

 outlets at the cloaca. The Eel closely resembles the Lamprey in 

 the general form and condition of the male organs; but the right and 

 left sides of the plicated testis are more distinct, and the spermatic 

 cells are more numerous and minute. 



The Sand-Eel {Amynodytes, No. 2378.) has a single testis, com- 

 pacted into an elongated triedral form, and impressed by a median 

 longitudinal fissure : it usually inclines a little to the right side. In 

 the Perch the single testis inclines to the left ; in the Blenny and 

 the Loach it lies in the middle line. In these osseous fishes the 

 glandular part of the testis is inclosed in a proper fibrous capsule, 



* Sir E. Home and Mr. Bauer, misled by this close resemblance to an ovarium, 

 inferred the identity of the testis with that body, described the kidneys as the testis, 

 and the Lampreys as hermaphrodite fishes. Hunter had recognised the true 

 structure, (vii. t. iv. pp. 204 — 206. pi. 59. fig. 1. /t.) Mr. Bauer gives a good 

 microscopic view of the cellular structure of the testis in Phil. Trans. 1828, pi. xv. 



