EVOLUTION-PAST AND PRESENT 



633 



Fig. 25-9. The early embryos of all vertebrates are very similar during some stage of their development. 



example, Volvox) resemble the blastula, and 

 the gastrula parallels the two-layered coe- 

 lenterates (for example, Hydra). Subse- 

 quent stages of higher embryos become 

 worm-like tubes reminiscent of the annelids 

 (for example, Neanthes). Organ systems 

 are formed in a primitive stage and function 



as such in primitive forms, while in higher 

 forms they are laid down only in basic de- 

 sign, to become something far more com- 

 plex at a later time. From this point on, the 

 embryo undergoes further changes which 

 take it farther and farther up the line of 

 animal development. Early embryos from 



