REPRODUCTION AND DEVELOPMENT 



249 



essentially young animals, grow larger and again the phenomenon 

 of fission occurs. 



In forms like Planaria velata and Planaria dorotocephala almost 

 any region of the body may serve as a reproductive mass, under- 

 going, when separated from the animal, the changes necessary to 

 develop a normal adult. In fact, in some flatworms exceedingly 



Fig. 171. — Twinning in a salamander embryo brought about by dividing the early 



embryo by a knot. (After Spemann.) 



minute portions possess this property. This is not a universal at- 

 tribute of the phylum, however, for some Platyhelminthes do not 

 undergo fragmentation or fission, reproducing solely sexually. 



The property of reproducing an adult from a fragment is also 

 found among Annelida and Echinodermata, but is absent in Mol- 

 lusca and in adult Arthropoda. It occurs in some of the lower 

 chordates, however. The property is entirely absent in the adult 

 vertebrate but a type of agamic reproduction may occur in the 

 embryo. The embryos of some vertebrates may be divided experi- 



