PRESIDENTIAL ADDRESS. 19 



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 ventricular septum in the heart of higher 

 vertebrates before the auricular septum is a well-known 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. 1 



But the most important cases in reference to distortion in time con- 

 cern the reproductive organs. If development were a strict and 

 correct recapitulation of ancestral history, then each stage would 

 possess reproductive organs in a mature condition. This is not the 

 case, and it is clearly of the greatest importance that it should not be. 

 It is true that the first commencement of the reproductive organs may 

 occur at a very early larval stage, or even that the very first step in 

 development may be a division of the egg into somatic and reproductive 

 cells ; and it is possible that, as maintained by Weismann, this latter 

 condition is a primitive one. Still, even in these cases the repro- 

 ductive organs merely commence their development at these early 

 stages, and do not become functional until the animal is adult. 



Exceptionally in certain animals, and as a normal occurrence in 

 others, precocious maturation of the reproductive organs takes place, 

 and a larval form becomes capable of sexual reproduction. This may 

 lead to arrest of development, either at a late larval period as in the 

 Axolotl, or at successively earlier and earlier stages, as in the 



1 Cf. Oldfield, Thomas. ' On the Homologies and Succession of the Teeth 

 in the Dasyuridfe, with an attempt to trace the history of the evolution of 

 the Mammalian teeth in general.' Phil. Trans. 1887. 



