The Development of Theridium. 349 



crease in length of the thoracic limbs; these fairly rigid cylinders 

 grow in length and thickness, press on the one hand against the 

 strong chorion of the egg, on the other against the lateral walls of 

 the thorax, and since these limbs are not capable of much bending 

 they would press the lateral walls of the embryo dorsad. This pres- 

 sure would tend to narrow the extraembryonic area and thereby to 

 push the abdomen out of its dorsal position. 



Further, the great widening of the ventral sulcus at the same time 

 shortens this sulcus, and this shortening would exert a pull upon those 

 margins of the abdomen bordering this sulcus. Throughout these 

 changes the yolk mass behaves passively, and in Tlieridium does not 

 j)roduce a particularly prominent ventral sac. Thus, the main factor 

 in reversion seems to be the lengthening of the thoracic appendages 

 and the pressures they exert upon the lateral walls of the body. 



LITERATURE LIST. 



Balbiani, E. G., 1873. M§moires sur le d§veloppement des Araneides. Ann. 

 Sci. Nat, 18. 



Balfouk, F. M., 1880. Notes on the Development of the Araneiua. Quart. 

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Babrois, J., 1878. Sur le developpement des Araignees. Journ. Anat. et 

 Physiol., J4. 



Beauee, A., 1894. Beitrage zur Kenntnis der Entwicklungsgeschiehte des 

 Scorpions. Zeit. wiss. Zool., 57. 



Bruce, A. T., 1886. Observations on the Embryology of Insects and Arach- 

 nids. J. Hopkins. Univ. Circ. 5. Ann. Mag. N. H. (5), 18. 



1886. Observations on the Embryology of Spiders. Amer. Nat., 



20. 



1887. Observations on the Embryology of Insects and Arachnids. 



Baltimore. 



CLAPARi:DE, E., 1862. Recherches sur revolution des Araignees. Utrecht. 



CoNKLiN, E. G., 1902. Karyokiuesis and Cytokinesis in the Maturation, Fer- 

 tilization and Cleavage of Crepidula and other Gasteropoda. Journ. 

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Croneberg, a., 1880. Ueber die Mundteile der Arachniden. Arch. Naturg., 46. 

 Jaworowski, a., 1891. Ueber die Extremitaten bei den Embryonen der 

 Arachniden und Insekten. Zool. Anz., 14. 



1892. Ueber die Extremitaten, deren Driisen und Kopfsegmentirung 



bei Trochosa singoriensis. Zool. Anz., 15. 



