226 CLASS XV. 



changed^ In the other reptiles the emhyro leaves the o^^g in a much 

 more perfect state. The development of the young in the egg is 

 effected, according to different external circumstances of temperature, 

 &c., in different periods of time. In many serpents and lizards the 

 development begins before the ^gg is laid, in the body of the parent, 

 and in some the membrane of the tgg is broken by the young one 

 before birth. In the tortoises, on the contrary, development begins 

 after the Qgg has been laid, and is completed only after the lapse of 

 several weeks ^. 



Let us now consider the organs of animal life. In the central 

 nervous system we remark that the spinal marrow is constantly 

 large in comparison with the brain. From the great length which 

 it has in serpents it is very thin, whilst, on the contrary, in frogs, 

 whose spinal column consists of only a few vertebrae, with remark- 

 able shortness, it is much broader. In the middle of the spinal 

 cord, as in fishes, there runs a canal which is continued into the 

 foui'th ventricle of the brain. Where the nerves for the limbs arise, 

 a swelling or expansion of the cord is seen corresponding to the 

 size of the limbs; in frogs, for instance, this swelling is very con- 

 spicuous at the place where the lumbar nerves arise for the hinder 

 limbs ^. 



The brain still forms in reptiles only a small part of the weight 

 of the body. According to the statements of Haller and Cuvier 

 (see the places quoted above p. 42) the ratio in the frog is tts, in 

 the ringed snake {coluber natrix) rk», in a land tortoise 22V0, in a 

 turtle, according to Caldesi, twws. The numbers given are sufficient 



•^ Compare, besides the works already quoted in the preceding note, Von Baer 

 Ueber EntwicJcelungsgescMchte der Thiere, ii. 1837, s. 280 — 295; K. B. Eeichert Das 

 Entwickelungsleben im Wirbelthier-Eeich, Berlin, 1840, 4to, s. 5 — 85; and C. Vogt 

 Untersuchungen ilher die EntwicJcelungsgeschichte der Geburtshelferhrote, Solothurn, 

 1842, 4to. 



2 We cannot here unfold the history of development. Besides the contributions to 

 it in earlier works (as for the development of lizards by Emmert and Hochstetter, 

 Reil's Archiv. x. s. 84 — 122, of tortoises in the work of the celebrated Tiedemann, 

 Zii S. T. Von S(emmerring's Juhelfeier, 1828, 4to), this subject has given occasion to 

 two excellent works by Eathke in later times; EniwickelungsgescJiichte der Natter, 

 Konigsberg, 1839, 4*'0j and Ueber die Entwickehing der Schildkroten, Braunschweig, 

 1848, 4to. 



3 Carus VersxLch einer Darstellung des Nervensystems und insbesondere des Gehirns. 

 Leipzig, 1814, 4to, s. 170 — 174, Tab. Iil. figs, i, 2 c, the spinal marrow of the frog; 

 see for the tortoise Bojands 1. 1. Tab. xxi. figs. 83, 84, p. 87. 



