816 COMPARATIVE ANATOMY 



simple and composed of solid bands or are sac-like, having an internal 

 lumen. In the simple forms the eggs pass into the coelom and thence to 

 the exterior by abdominal pores or by oviducts of varying lengths. We 

 do not know whether these ducts are true Miillerian ducts or whether 

 they are new formations. 



The sacular condition of the ovaries may come about by the free 

 edge of the ovary bending laterally and fusing with the wall of the 

 coelom. This forms a cavity called the parovarial canal, closed in front. 

 Or there may be a groove in the covering epithelium forming on the 

 surface of the ovary. . In this case, as it closes and sinks inward, it forms 

 what is called an entovarial canal. In either case the canal may extend 

 to the most caudal end of the body cavity and form an oviduct in this 

 manner, "or the oviduct may be formed from both kinds of canals, one 

 in front, the other behind." 



"From this it would appear that the ovary originally extended back 

 to the hinder end of the coelom (as it does in Cyclopterus) or that the 

 par- or entovarial canal had united with a Miillerian duct which has 

 otherwise been entirely lost. The oviducts thus formed usually unite 

 before opening to the exterior, either directly or via a urogenital sinus." 



It will be remembered that there are shell glands (likewise called 

 nidamental glands) in those animals which are oviparous, although these 

 may appear in viviparous forms also, though they are very slight in 

 these latter instances. It is interesting to note that in some species of 

 elasmobranchs the eggs are larger than those of an ostrich. In this same 

 type of animal the caudal or inner side of the pelvic fin is specialized 

 for a copulatory organ, for in the elasmobranch fertilization is internal. 

 In the amphibia there are many interesting accessory reproductive re- 

 lations as mentioned in the chapter on classification of vertebrates. The 

 caecilians and Amphiuma lay their eggs in long strings in the soil and 

 the female incubates them although the male often takes charge of the 

 eggs. In Pipa each egg undergoes development in a pit in the skin of the 

 back of the female, and in Nototrema and Opisthodelphys (South 

 American tree-toads), there is a large pocket in the skin of the back, 

 opening near the coccyx, where the eggs are carried until partially 

 (Nototrema) or entirely developed. Salamandra maculosa and S. atra 

 bring forth living young, the former possessing gills at birth, the latter in 

 the adult form. 



In the higher forms of vertebrates there is a definite single copula- 

 tory organ. Among the sauropsida the Sphenodon. alone lacks all copu- 

 latory organs, while in most birds they are incomplete. The males of 

 crocodiles, turtles, ostriches, ducks, geese, and swans are among the very 

 few that have a definite structure homologous to that of mammals for 

 this purpose. In snakes and lizards several sacs are developed from the 

 caudal wall of the transverse anus. They resemble appendages in the 

 embryo and form real copulatory organs called hemipenes. They are 

 present in both sexes though very small in the female. As growth con- 



