TRACIIEATA. 377 



and segments, very considerable differences are noticeable in the different 

 Arachnoid types. This fact alone appears to me to be sufficient to prove 

 that the order of appearance of the appendages is often a matter of 

 embryonic convenience, without any deep morphological significance. In 

 Scorpio the segments develop successively, except perhaps the first post- 

 oral, which is developed after some of the succeeded segments have 

 been formed. In Spiders the segment of the chelicerse, and probably also 

 of the pedipalpi, appears later than the next three or four. In both these 

 types the segments arise before the appendages, but the reverse appears to 

 be the case in Chelifer. The permanent appendages, except the chelicerse, 

 appear simultaneously in Scorpions and Spiders. The second pair appears 

 long before the others in Chelifer, then the third, next the first, and 

 finally the three hindermost. 



BIBLIOGRAPHY. 



Scorpionidce. 



(434) El. Metschnikoff. " Erubryologie des Scorpions." Zeit. f. wiss. Zool. 

 Bd. xxi. 1870. 



(435) H. Eathke. EeisebemerUungen aus Taurien (Scorpio), Leipzig, 1837. 



Pse udoscorpion {dee. 



(436) El. Metschnikoff. " Entwickluugsgeschicbte d. Chelifer." Zeit.f. iviss. 

 Zool., Bd. xxi. 1870. 



(437) A. Stecker. "Entwicklung der Chtkonius-Eier im Mutteiieibe mid die 

 Bildung des Blastoderms." Sitzung. konigl. biihmisch. Gesellscliaft Wissemch., 1876, 

 3. Heft, and Annul, and Mag. Nat. History, 1876, xvm. 197. 



Phalangidce. 



(438) M. Balbiaui. "Memoire sur le developpemeut des Phalangides." Ann. 

 Scien. \at. Series v. Vol. xvi. 1872. 



Araneina. 



(439) M. Balbiani. "Memoire snr le developpement des Araneides." Ann. 

 Scien. Nat. Series v. Vol. xvn. 1873. 



(440) F. M. Balfour. "Notes on the development of the Araneina." Quart. 

 Journ. of Micr. Science, Vol. xx. 1880. 



(441) .1. Barrois. " Recherches s. 1. developpement des Araignees." Journal de 

 VAnat. et de la Pliysiol. 1878. 



(44-2) E. Claparede. Eecherches s. revolution des Araignees. Utrecht, 1862. 



(443) Herold. De generatione Araneorum in Ovo. Marburg, 1824. 



(444) H. Ludwig. "Ueber die Bildung des Blastoderms bei den Spinnen." 

 Zeit.f. wiss. Zool., Vol. xxvi. 1876. 



Acarina. 



(445) P. van Benedeu. ''Developpement de 1'Atax ypsilophora." Acad. Bru- 

 t. xxiv. 



(446) Ed. Claparede. " Studien iiber Acarinen. " Zeit.f. iviss. Zool., Bd. xvm. 



Formation of the layers and the embryonic envelopes in tlie Tracheata. 



There is a striking constancy in the mode of formation of the 

 layers throughout the group. In the first place the hypoblast is not 



