106 ON THE ORIGIN AND [CHAP. 



first a single globular mass, the first changes which take 

 place in it being as follows : " The central nucleus 

 becomes drawn out and subdivides into two, this 

 division being followed by a corresponding segmenta - 

 tion of the yolk. The same process is repeated again 

 and again, until at length the entire yolk is converted' 

 into a mass of minute cells." Among the Crustacea 

 the total segmentation of the yolk occurs among the 

 Copepoda, Rhizocephala, and Cirripedia. Sars has 

 described the same process in one of the nudibran- 

 chiate mollusca 1 (Tritonia), Muller in Entochocha, 2 

 Haeckel in Ascidia, 3 Lacaze Duthiers in Dentalium. 4 

 Figures 1 8 to 21, PI. 6, are taken from Koren and 

 Danielssen's 5 memoir on the development of Purpura 

 lapillus. 



Figs. 22 24 show the same stages in a fish 

 (Amphioxus) as given by Haeckel, and it is unneces- 

 sary to point out the great similarity. 



Lastly, figures 25 to 29, PI. 6, are given by Dr. 

 Allen Thomson, 6 as illustrating the first stages in the 

 development of the vertebrata. 



I might have given many other examples, but the 

 above are probably sufficient, and will show that the 

 processes which constitute the life-history of the 

 lowest organized beings very closely resemble the 

 first stages in the development of more advanced 



1 Wiegmann's Archiv., 1840, p. 196. 



2 Ueber die Erzeugung von Schnecken in Holothurier. Berlin, Bericht, 

 1851. Ann. Nat. Hist. 1852, v. ix. M tiller's Archiv., 1852. 



3 Natiirliche Schopfungsgeschichte, pi. x. 



4 Ann. des Sci. Nat 1853, p. 89. 



5 Ann. des Sci. Nat. 1857, pi. vi. 



6 Cyclopaedia of Anatomy and Physiology. Art. Ovum, p. 4. 



