140 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



inal cavity, but are imicli flattened or compressed laterally, instead of 

 cylindrical, as in the former, and botli pour tlieir secretion tlirougli a 

 wide seminal duct which opens behind the vent. Their substance is 

 comx)osed almost entirely of a vast number of minute canals, which 

 have a generally vertical or oblique direction, and which open into a 

 wide sinus or space at the upi)er edge of the organs, and which empty 

 their contents into the common seminal duct. They are essentially 

 glandular in function, and secrete and pour out large quantities of milt 

 or semen during the spawning season, which is develoj)ed in the tubules 

 or canals already alluded to. These canals, or, more properly, semi- 

 niferous tubules, are lined with cells, which break up into bundles of 

 spermatozoa, which fall directly into their cavities, and so find their 

 way into the seminal sinus at the upper border of the testicle and out 

 through the seminal duct into the water, where they are capable, if 

 they come into contact with the ova discharged by the female, of effect- 

 ing impregnation and of establishing the development of the embryos. 

 The spermatic particles, or spermatozoa, are themselves very minute, 

 and are composed, in the mackerel, of an oval head with a very fine, 

 almost ultra-microscopic, tail or flagellum, which is incessantly lashed 

 about in the living state, so that the spermatozoan has a distinctive 

 wriggling, tadpole-like movement. As soon as the power to move the 

 lash or flagellum ceases, they may be considered as dead, and no longer 

 capable of effecting the impregnation of ova. They will not ordinarily 

 live much over an hour after being taken from the fish, with which 

 time their eflfectiveness ceases. 



STRUCTURE AND PHYSIOLOGICAL CHARACTERISTICS OF THE UNFER- 

 TILIZED EGG OF THE SPANISH MACKEREL. 



It is notorious that the egg-membranes of floating fish ova are ex- 

 tremely thin; moreover, they are not, as far as I have been able to make 

 out with carefully conducted observations, perforated by pore-canals, as 

 in the stickleback, salmon, and shad; the membrane of the ovum of the 

 mackerel is no exception to this rule, and is conseqT,iently not a zona ra- 

 diata, as defined by Balfour. It is a perfectly houiogeneous, transparent 

 film, less than half as thick as that covering the shad ovum, which meas- 

 ures approximately ^-oVo of au inch in thickness, but differs from the 

 latter in having minute i^apular iiromineuces on the inner surface which 

 project into the breathing chamber or water space around the germ, as 

 shown in Figs. 2, 6, 9, and 12. These prominences usually seem to be 

 confined to one pole or hemisphere of the membranous envelope. 



The ova vary slightly in dimensions and range from -^ to ^o of ^^ 

 inch in diameter. This variation in size is a usual feature in the ova of 

 fishes, but may be partly due to the unusual pressure exerted on the 

 ovaries when the ova are removed artificially, so that some are squeezed 

 from their follicles before they are quite fully mature, though it is to be 

 remarked that the smaller ova develop just as readily as the larger 



