LECTURES. 63 



Will someone explain the difference between analogy 

 and homology? Of embryology as a study in itself 

 Professor Agassiz said: "It is a wonder that such 

 broad and comprehensive generalizations could have 

 been made upon a basis of knowledge derived from 

 so few animals." 



Again, he takes up embryology and says: 

 "The eggs of birds were known, and their parts 

 named, very early; these have been transferred to all 

 other eggs. We have the yolk, with its vitelline 

 membrane; the white; the shell with its two lining 

 membranes with the air space between them at the 

 larger end; and the suspensory cords. Of all these 

 the yolk alone is necessary in producing the young, 

 in fact, the others may all be wanting. Eggs can be 

 of any form. The yolk is a fluid, organized, and at 

 first appears albuminous. The blastoderm, or life 

 portion, is surrounded by a congregation of light cells 

 which cause it to always appear on the top no mat- 

 ter how the egg may be placed. Baer discovered the 

 mammalism egg, and about the same time, 1837, the 

 cell doctrine was advanced. Through the influence 

 of Schleiden the structure of animals and plants be- 

 gan to be compared. Studies of cells and the cellu- 

 lar structure has been continued ever since. They 

 described the cell membrane, the nucleus or point of 

 special life, and the nucleolus or point within this 

 point. Microscopists sought cell tissue everywhere. 

 Embryologists sought the smallest eggs. All parties 

 agreed, finally, that eggs were cells destined to an en- 

 larged growth a peculiar development and, ultimate- 

 ly, an individual existence. 



"Forty years ago the theme of science was the 

 function of organs: Today, it is cells. The minute 

 tentacle of the hydroid polyp contains, at once, cells 

 nervous, muscular, and assimulative. Thus all 

 structures are formed of differentiated or specialized 

 cells; all parts of animals are formed of cells. The 

 study of the changes undergone by these cells has 

 only just begun. Until we know how new individ- 



