THE RECAPITULATION THEORY 317 



essential for them to hatch in a condition capable 

 of independent existence i.e., capable at any rate 

 of obtaining and digesting their own food. 



Anachronisms, or actual reversal of the historical 

 order of development^ of organs, or parts, occur 

 frequently.^ Thus the joint surfaces of bones 

 acquire their characteristic curvatures before move- 

 ment of one part or another is effected, and before 

 even the joint cavities are formed. Another good 

 example is afforded by the development of the 

 mesenterial filaments in Alcyonarians. Wilson has 

 shown in the case of Renilla that in the develop- 

 ment of an embryo from the egg the six endodermal 

 filaments appear first, and the two long ectodermal 

 filaments at a later period ; but that in the forma- 

 tion of a bud this order of development is reversed, 

 the ectodermal filaments being the first formed. 

 He suggests in explanation, that as the endodermal 

 filaments are the digestive organs, it is of primary 

 importance to the free embryo that they should be 

 formed quickly. The long ectodermal filaments 

 are chiefly concerned with maintaining currents of 

 water through the colony ; in bud-development 

 they appear before the endodermal filaments, 

 because they enable the bud during its early stages 

 to draw nutrient matter from the body fluid of the 

 parent ; while the endodermal filaments cannot 

 come into use until the bud has acquired both 

 mouth and tentacles. The completion of the ven- 

 tricular septum in the heart of higher vertebrates 

 before the auricular septum is a well-known 

 anachronism, and every embryologist could readily 



