ANIMAL PEDIGREES 227 



ectodermal filaments at a later period ; but that in 

 the formation 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 its 

 own mouth and tentacles. 



The completion Of the ventricular septum in the 

 heart of higher vertebrates before the auricular 

 septum is an often quoted anachronism, and every 

 embryologist could readily furnish many other 

 cases. A curious instance is afforded by the 

 development of the teeth in mammals, if recent 

 suggestions as to the origin of the milk dentition 

 are confirmed, and the milk dentition prove to be a 

 more recent acquisition than the permanent one.* 



Distortion of a curious kind is seen in cases of 

 abrupt metamorphosis where, as in the case of 

 many Echinoderms, of Phoronis, and of the 

 metabolic insects, the larva and the adult differ 

 greatly in form, habits, mode of life, and very 

 usually in the nature of their food and the mode of 

 obtaining it ; and the transition from the one stage 

 * This has since been disproved. ED. 



