402 : FISHES CHAP, 
Salmonidae,’ where they are as singularly variable in different 
species and individuals as in the Elasmobranch Scylliidae. The 
use of abdominal pores is not certainly known, unless the coelom of 
those Fishes which possess them continues to retain some measure 
of its primitive excretory function, and the pores act as excretory 
ducts. That the nephrostomes are excretory organs has been 
shown by experiment, and it is worthy of note that there exists a 
reciprocal relation between these structures and abdominal pores, 
to the extent that while there are a few Fishes (e.g. certain 
Elasmobranchs and Amia) in which both coexist, there are 
many others in which the presence of nephrostomes is correlated 
with the absence of pores and vice versd. 
The male and female gonads, testes and ovaries, are derived 
from the coelomic epithelium near the inner or median aspect of 
the nephrotomes (Fig. 229, B). Here the epithelium remains 
columnar, and soon projects into the ventral coelom as a con- 
tinuous longitudinal ridge. It is probable that at first the modified 
epithelium is segmented as a series of “ gonotomes,” but if so, the 
latter must soon coalesce into a continuous ridge. Some of the 
epithelial cells enlarge to form the primitive sex-cells. In the 
development of an ovary, portions of the epithelium sink inwards, 
carrying with them the primitive ova. Certain of the cells form 
the epithelial walls of a number of ovisacs, each of which encloses 
an ovum. As the ovisacs increase in number and size the 
germinal ridges project more and more into the coelom until, 
as ripe ovaries, they become suspended from its dorsal wall by 
a double peritoneal fold, the “ mesovarium” (Fig. 156). The testes 
develop in a similar fashion except that the primitive sex-cells, 
which later give rise to spermatozoa, form the lining of a number 
of simple or ampulla-like tubules, the seminiferous tubules, and 
the suspensory fold is termed the “ mesorchium.” 
The Cyclostomes have gonads in the shape of unpaired 
organs extending nearly the whole length of the coelom, but 
in all Fishes the organs are primarily paired, although by fusion, 
or by the absorption of one gonad, the ovaries or the testes 
sometimes appear as if single. The ovaries may either be naked, 
as in Elasmobranchs, Dipnoi, Crossopterygu, and Chondrostei, 
and in Amia amongst the Holostei; or, as in Lepidosteus and 
most Teleosts, they become enclosed in coelomic sacs. The 
1 Max Weber, Morph. Jahrb. xii. 1886, p. 336. 
