PRESIDENTIAL ADDRESS. 17 



got rid of the free swimming ciliated larval stage, and which leave the 

 egg of considerable size and strength, that can establish themselves as 

 fresh-water animals. This is effected most readily by the acquisition of 

 food yolk — hence the large size of the eggs of fresh- water animals — and 

 is often supplemented, as Sollas has shown, by special protective devices 

 of a most interesting nature. For this reason fresh-water forms are not 

 so well adapted as their marine allies for the study of ancestral history 

 as revealed in larval or embryonic development. 



Before leaving the question of food yolk, reference must be made to 

 the proposal of the brothers Sarasin, to regard the yolk cells as forming 

 a distinct embiyonic layer, the lecithoblast^distinct from the blastoderm. 

 I do not desire to speak dogmatically on a point the full bearings of 

 which are not yet apparent, but I venture to think that this suggestion 

 will not commend itself to embryologists. The distinction between the 

 yolk granules and the cells in which they are imbedded is a real 

 and fundamental one ; but I see no reason for regarding the yolk cells 

 as other than originally functional endoderm cells in which yolk 

 granules have accumulated to such an extent that they have in extreme 

 cases become devoted solely to the storing of food for the embryo. 2 



Of all the causes tending to modify development, tending to obscure 

 or falsify the ancestral record, food yolk is the most frequent and the 

 most important ; its position in the egg determines the mode of 

 segmentation ; and its relative abundance affects profoundly the entire 

 embryonic history, and decides at what particular stage, and of what 

 size and form, the embryo shall hatch. 



The loss of food yolk is another disturbing element, the full influence 

 of which is as yet imperfectly understood, but the possibility of which 

 must be always kept in mind. It is best known in the case of mammals, 

 where it has led to apparent, though very deceptive, simplification of 

 development ; and it will probably not be until the embryology of the 

 large-yolked monotremes is at length described, that we shall fully 

 understand the formation of the germinal layers in the higher placental 

 mammals. 



Amongst invertebrates we know but little as yet concerning the 



1 P. and F. Sarasin, Ergebnisse naturivissenschaj flicker Forschungen an/ 

 Ceylon. Bd. ii. Heft iii. 1889. 



9 Cf. E. B.Wilson, 'The Development of Kenilla.' Phil. Trans. 1883. 

 p. 755. 



B 



