172 AGE, GROWTH, AND DEATH 



process was first observed by some French investiga- 

 tors, 1 because it is so descriptive of the appearance 

 presented to the eye by the changes which are going 

 on. Were we to name the process now we should 

 certainly call it a process of cell production. 



The next of our pictures (Fig. 60) shows us the 

 eggs of a common animal, the Planorbis, a little 

 fresh-water snail, the coils of which lie flat in one 

 plane hence its name. 2 No. i shows the original 

 germ, which has an actual diameter of 0.132 $mm.; 

 No. 2 shows it about to divide into two. No. 3 is a 

 side view and No. 4 a top view of the ovum with two 

 segments ; No. 5 is cleft into four segments ; No. 6 

 into eight. Nos. 7 and 8 illustrate the further pro- 

 gress of the cell multiplication ; No. 9 represents the 

 under side of the same egg, of which the top is figured 

 as No. 8. The number of cells (segments) is thus 

 constantly increasing, and already it is evident that 

 they have become somewhat unlike in character. 

 The pictures were made from the living egg, and 

 therefore do not give satisfactory views of the nuclei, 

 but nevertheless there is during segmentation a 

 change going on in them, which, however, I can bet- 

 ter demonstrate to you by means of Fig. 58. Taken 

 from sections through the early developing germ 



1 Especially Prevost and Dumas, " Developpement des ceufs des batraciens," 

 Annales des sciences naturelles, 1842, Tome II., pp. 100 and 129, and earlier 

 papers. 



2 The figures are copied from Carl Rabl's classic memoir, a fine monument 

 of capable and thorough research ; see " Ueber die Entwickelung der Teller- 

 schnecke," Morfkologisckes Jahrbuch, v., 561-655, Taf. XXXII.-XXXVI1I, 



