DEVELOPMENT. 



185 



alone have eopulatcry organs in the form of the swollen lips of the 

 cloaca, which during copulation clasp the cloaca! aperture of the 

 female, and thus render an internal fertilization possible. In this 

 case the eggs can undergo their development within the body of the 

 female, and the young be born at a more or lees advanced &tage of 

 development. 



It is only in exceptional cases that the parents have an instinct 

 which leads them to watch over the further fate of their brood, as 

 for example Alytes (fig. 626) and the South American Surinam 

 Toad (Pi/pa dorsigera). The male of Alytes winds the string of eggs 

 round its hind legs and burrows into the damp earth, and only gets 

 rid of his load when the embryonic development is completed. The 

 male of Pipa places the eggs when laid on the back of the female, 

 which then develops a cell-like pouch round 

 each egg. The larvae are hatched and undergo 

 their metamorphosis in these pouches. In 

 other genera, as Notoddpliys, the females 

 possess a spacious brood-sac beneath the dor- 

 sal integument. Except in these cases, the 

 eggs are either attached singly to water plants 

 (Tritonidce) or laid in strings or irregular 

 clumps. 



Development. The eggs, which are rela- 

 tively small,* Undergo an unequal segmenta- 

 tion (vol. i., fig. 104) after fertilization. The 

 Amphibia agree with the Fishes in not 

 developing an amnion or allantois the 

 embryonic membranes of such importance in the higher Verte- 

 brates. In the Amphibians, however, the urinary bladder which 

 arises from the ventral wall of the cloaca is morphologically equiva- 

 lent to the allantois. The embryos are also without any external 

 yolk-sac constricted off from the body, the yolk being enclosed at an 

 early period by the ventral plates. As respiratory organs gills are 



* C. E. v. Baer, " Ueber die Entwickelungsgeschichte der Thiere," II., 

 Konigsberg, 1837. 



Reichert, " Das Entwickelungsleben im Thierreich," Berlin, 1840. 



C. Vogt, " Untersuchungen iiber die Entwickelungsgeschichte der Geburts- 

 helferkrote," Solothurn, 1842. 



Rusconi, " Histoire naturelle, developpement et metamorphose de la Sala- 

 mandre terrestre," Paris, 1854. 



A. Gb'tte, "Entwickelungsgeschichte der Unke," Leipzig, 1874. 



0. Hertwig, " Die Entwickelung des mittleren Keimblattes der Wirbelthiere," 

 Jt-n. HfttHrwtes. Zeitscltr.. Tom. XV., 1881. 



Fi. 626. Alytes 

 Male with the string of eggs. 



