THE PRESIDENT’S ADDRESS. 193 
He next traces with some minuteness the process of yolk-segmen- 
tation in the egg, and shows that there is a strict analogy between 
the development of the embryo of the bird and of the mammal; 
and finally concludes that a survey of the whole animal kingdom, 
from the simplest animal up to man himself, will show that all the 
various modifications of organic structure which present themselves, 
spring originally from two cellular lamine, the ectoderm and ento- 
derm; the component elements of which may be again traced back 
to the first segment sphere and primitive protoplasmic elements of 
the ovum. 
The investigation thus indicated is of the highest interest, and 
we follow our guide with ever-increasing pleasure as he builds up, 
layer by layer, the wonderful structure of the animal frame. In 
this, the ontogenetic history of animals, as he calls it, all must 
remain willing disciples at their master’s feet. But when he passes 
from the history of the individual to the phylogenetic, or race- 
history of the formation of animals and man, the master’s theory 
may perchance seem, even to willing disciples, to be running ahead 
of his facts, and his generalisations to be made too rapidly. 
His argument is somewhat as follows: If the changes by which 
the complex organization of the body is gradually built up out of 
its elementary materials, in individuals belonging to different classes 
and orders of animals, are found to be not only not different, but on 
the contrary, to present features of the most remarkable resemblance 
and conformity, then we may conclude that there is a general plan 
of development extending to the members of considerable groups, 
and possibly capable of being traced from one group to another. 
‘‘ But this is clearly nothing less than another way of stating that 
there is a similar type of structure pervading the animals of each 
group, and a probability of a common type being ascertained to 
belong to them all.” 
Illustrations are then given of this remarkable correspondence 
in gradation and development of type in animals of different orders. 
1. From the process of incubation in the chick. 
2. From the formation of the chorda dorsalis. This exists as a 
primordial structure in the embryo of all vertebrates, including man 
himself, and is to be found among the invertebrates in the larva of 
the Ascidia—the latter being a mollusc without head or shell, 
having two orifices nearly on a level, and shaped like a leathern 
bottle (hence its name, from Greek doxds), haying no motion beyond 
