THE MOST PRIMITIVE METAZOA 



61 



Fig. 20. Diagram to show inversion during the development of 

 daughter cells in Volvox. The flagella of the daughter colony all 

 point towards the inside of the colony. Inversion takes place and the 

 colony turns inside out so the flagella now point outwards. (After 



Zimmerman.) 



In both Volvox and Grantia the cells develop in the same initial 

 relationship to the embryo as they do to the parent layers. Hyman 

 (1940) was impressed by the similarity between the inversion in 

 Volvox and the sponges and suggested that this might indicate a 

 common ancestry. On the other hand Tuzet (1945) considered 

 the situation as one of convergence which does not indicate any 

 underlying phylogenetic relationship. 



