342 



months by the press of other work. I wish to express my obligations 

 to Mr. G. T. Cline for the figures illustrating this paper. 



In the development of R. pipieus, the mesentery is formed when 

 the larva has a total length of 7.5 mm. Prior to this time, the dorsal 

 wall of the archenteron is in contact with the aorta, while the lateral 

 plates of mesoderm lie at some distance on either side of the median 

 line. Fig. 1. This figure shows the relation of these parts shortly 

 before the formation of the mesentery. It will be seen that the endo- 

 derm is elevated to form a ridge of cells between the two lateral 

 plates of mesoderm. The cells constituting a large portion of this 

 ridge are the future sex-cells. 



— S.N. 



Ao. 



— S.C.R. 



i, M.P. 



e.V. 



Arch. 



Fig. 1. A portion of a transverse section of a larva of Rana pipiens of 7 mm 

 total length. C.V. Cardinal vein. 3I.P. Lateral plates of mesoderm. S.C.R. Sex-cell 

 ridge. Arch. Archenteron. Ao. Aorta. S.N. Subchorda. JV. Notochord. 



