324 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



nephrostomes. Then these flow together, forming a large opening, 

 the ostium tubae 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 tubae 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 tubae (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 ccelom and thence 



