380 



Rathke's material (Alligator lucius, A. sclerops and A. cyno- 

 cephalus) does not seem to have been preserved in a manner favourable 

 to these structures, for although he notices their presence in some 

 cases, they show a tendency to be absent in his older specimens (as 

 in the preserved mammalian foetuses mentioned above). He could not 

 find them in lizard or chick embryos — no doubt because he also 

 was looking for expanded structures like in the Crocodilia. 



From GöLDi's short description we can gather that the two 

 Amazonian species of Caiman are provided with neonychia, probably 

 resembling the Tolypeutes type. 



Summary. 

 The claws of adult unguiculate Vertebrata consist of two principal 

 parts, the "Krallenplatte" and "Krallensohle" as established by Boas. 

 In their embryos a third portion is present, called in this paper the 

 neonychium. These three portions are all local differentiations of a 

 single rudiment. The neonychium is an entirely provisional embryonic 

 structure. Its shape varies slightly in ditierent forms (Tolypeutes and 

 chick types) but its relations with the other parts of the claw are 

 always such that its presence turns the tip of the claw from a hard 

 scratching point into a smooth rounded surface. It is highly probable 

 that its function is to protect the embryonic membranes from being 

 torn by the claws during movements of the embryo. As the time for 

 birth or hatching approaches the neonychium begins to break away 

 from the rest of the claw, and is probably in most cases rubbed otf 

 by contact with the ground directly the embryo emerges, as was shown 

 to be the case in the chick. 



Literature. 



1) Boas, J. E. V., Ein Beitrag zur Morphologie der Nägel, Krallen, Hufe 

 und Klauen der Säugetiere. Morph. Jahrb., Bd. 9, 1883. 



2) — , Zur Morphologie der Wirbeltierkralle. Morph. Jahrb., Bd. 21, 1894. 



3) GöLDi, E. A,, Hufförmige Verbreiterungen an den Krallen von K^rokodil- 

 embryonen. Zool. Anz., Bd. 23, 1900. 



4) Hausmann, W., Ueber Bau, Wachstum und Entwickelung der Krallen 

 der Säugetiere, vorzüglich der Talpa europaea und des Dasypus 

 novemcinctus, Leipzig 1898. 



5) Krause, W., Die Entwickelung der Haut und ihrer Nebenorgane, 

 in Hertwigs Handbuch, 1906. 



6) Rathke, H., Untersuchungen über die Entwickelung und den Körper- 

 bau der Krokodile, 1866. 



7) VoELTZKOw, A., Biologie und Entwickelung der äußeren Körperfoi*m 

 von Crocodilus madagascariensis. Abb. Senckenb. Naturf. Gesellsch., 

 Bd. 26, 1899. 



