1896.] Principles of Geology and its Aim. 271 



of peritoneum. Posteriorly the sex ridges of the two sides are 

 united into a single ridge. There is considerable variation in 

 the rate of segmentation in larvse of the same size, but the fol- 

 lowing table will give an idea of the segmentation and the 

 number of cells in successive stages : 



10 28-183 6-9 



12 39-143 7-9 



15-17 638-2280 11-13 sexes distinct 



16-25 2200-8000 13-15 



The sexes can first be distinguished not by the differences 

 in the sex cells, but in the tunic of peritoneal cells. A small 

 groove on the outer ventral pert of the sex ridge is the first 

 indication of the ovarian cavity and the surest criterion of the 

 female. In the male the sex gland remains much more cir- 

 cular in cross section and no groove is developed. Much later 

 histological differences in the sex cells themselves can be made 

 out. The long slender chromatin threads of the female cell 

 just before dividing are represented in the male by short, thick 



(Continued from paye 183.) 

 Lamarck and Defrance earnestly engaged in study of fossil 

 shells, and the former, in 1802, reconstructed the system of 

 conch ology and introduced into it the new species collected 

 by the latter from the strata underlying the city of Paris and 

 quarried for the construction of its buildings. Six years pre- 

 vious to this Cuvier had established the different specific char- 

 acter of fossil and living elephants and he devoted himself to 

 researches throughout the remainder of his life. Jameson, 

 in 1808, pointed out the nature of all the rocks and the mode 

 in which they were formed, and made use of the observations 



