324 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
nephrostomes. Then these flow together, forming a large opening, 
the ostium tubz abdominale, on either side (in elasmobranchs the 
ostia of the two sides are usually united ventral to the liver) through 
which the eggs, which pass from the ovary into the ccelom are carried 
into the oviduct. 
In some amphibia (Salamandra) the pronephric tubules and neph- 
rostomes take a part in the formation of the ostium tube and the 
beginning of the oviduct, while in Amblystoma the ostium develops 
in close connection with the pronephric nephrostomes. Here, as in 
all other tetrapoda, the rest of the oviduct arises by the formation of a 
groove of the peritoneal membrane close beside the Wolffian duct. 
This becomes rolled into a tube, the Miillerian duct. In the amniotes 
the anterior, end of the groove does not close, but remains open as the 
ostium tube (fig. 321, A). : 
Usually the condition in the elasmobranchs has been regarded as 
the primitive one, a supposition which renders it difficult to homologize 
the Miillerian ducts (oviducts) of elasmobranchs with those of other 
forms. Still, when the adult conditions are considered—similar ostia, 
similarity of position, and of external openings—it is hardly possible 
to believe them as merely analogous, as examples of convergence. 
The facts in the amphibia, referred to in the preceding paragraph 
are additional evidence of homology. If, however, it be assumed 
that the more common type of development, by the infolding of cce- 
lomic epithelium, be the primitive condition, the difficulties are less, 
though not entirely solved. Then, if it be that the homologous tissue 
in the elasmobranchs was at first included in the tissue of the pro- 
nephric duct and that the splitting is a secondary operation to separate 
parts which elsewhere are always distinct, the similarities are more 
apparent. 
In the females, as in the males, of cyclostomes and teleosts the 
reproductive ducts are not easily brought into harmony with those 
of other vertebrates, and an answer to all questions cannot be had until 
the development of the parts has been studied in more forms, and 
especially the ganoids and dipnoi. In the cyclostomes the eggs are 
shed from the ovaries into the ccelom and are thence passed outward 
through the abdominal pores. 
In the teleosts there are several conditions. The ovaries may be 
simple and solid bands or saccular in character with an internal lumen 
(fig. 326, E). In the first the eggs pass into the coelom and thence 
