METAMORPHOSIS. 221 



acter. Let us consider Salpa as an example. In 

 this genus the fertilised egg develops into a small 

 transparent barrel-shaped creature, which swims free 

 and near the surface of the ocean. When the ani- 

 mal has attained its growth, there appears on the 

 ventral side of it a very small stem or rod-like 

 prolongation of the internal tissues, which is des- 

 tined to develop into a colony of individuals. This 

 stem, or stolon, as it is called, is made up of tubes 

 developed from the entodermal and mesodermal 

 tissues, covered externally by ectodermal tissue. 

 As the stolon becomes longer, its distal portion 

 enlarges and divides — without complete separation 

 — into a series of transverse segments. Each seg- 

 ment receives thus a part of each of the three 

 fundamental tissues. As the segments are increased 

 in size, they are squeezed alternately right and left, 

 though all remain connected by a common circula- 

 tion. Each segment develops into modified barrel- 

 shaped individuals with male and female productive 

 tissues. At length the oldest part of the stolon, 

 having developed thus into a colony, breaks loose 

 from the rest of the stolon, which, in its turn, fol- 

 lows the same course of development. The indi- 

 viduals of the now free-swimming colony proceed 

 to reproduce sexually the eggs from which the first 

 described single form of Salpa develop. One thing 



