60 



THE MOST PRIMITIVE METAZOA 



Fig. 19. Diagram to show inversion during the embryology of the 



sponge Grantia. The embryo develops at first with its flagella 



pointing inwards (1). The embryo slowly inverts (2-3) so that its 



flagella now point outwards. (After Duboscq and Tuzet.) 



end of the larva has the non-flagellated cells and after swimming 

 for some time the amphiblastula larva settles on the substratum, 

 the non-flagellated cells grow over the flagellated cells so that the 

 animal takes up the structure of an adult sponge. 



A similar inversion takes place during the development of the 

 daughter colonies of Volvox (Pocock 1933). During the asexual 

 development of a daughter colony the daughter cells divide and 

 form a sheet of cells. These cells are orientated in the same manner 

 as the parent cells, the flagella pointing outwards, but as division 

 proceeds the daughter cells form a ball with the flagella pointing 

 towards the centre of the ball. The colony is not a complete 

 ball since there is a hole at the top. The colony now proceeds to 

 turn itself inside out through this hole in much the same way as 

 one might push a tennis ball inside out through a hole in the 

 wall (Fig. 20). This results in a colony with the flagella all 

 pointing outwards, the inversion taking some two hours. 



