32 The Beginnings of Multicellular Organization 



REGULATION 



This term refers to the abihty of a morphogenetic system to turn out 

 normal products of different absolute sizes. For example, individual frog 

 embryos or tadpoles or adult frogs of a given species can vary markedly 

 in size and yet be perfectly proportioned. That is, the sizes of the various 

 parts (arms, legs, head, etc.) at each developmental stage bear precise 

 relationships to the total length or total mass. Furthermore, if cells are 

 surgically removed from a very young frog embryo, the remainder will 

 develop into a very tiny but nonetheless normal tadpole, again demon- 

 strating the same relationship of parts to whole. 



Slime molds also display regulatory capacities. For example, myx- 

 amoebae packed together in a solid mass produce relatively large migrat- 

 ing slugs and fruiting bodies; some are as much as 5 mm in length and 

 contain hundreds of thousands of cells. If sparsely distributed, the myx- 

 amoebae form tiny slugs and fruits a few tenths of a millimeter in length 

 that contain a few hundred cells. Yet the gross proportions and the rela- 

 tive amounts of cells comprising the various parts of the fruiting body 

 remain the same. This regulatory capacity is exhibited at a truly startling 



Fig. 16. Two views of a 

 fruiting body formed by 

 the "fruity" mutant of D. 

 discoideum. This highly or- 

 ganized multicellular struc- 

 ture consists of 9 spores, 2 

 stalk cells, and I basal cell. 



