98 THE MICROSCOPICAL NEWS. 
it in the opposite direction, (the systemic;) so that the heart is alter- 
nately a venous and an arterial chamber. An analogy has been 
found in the relation of the notochord to other parts in the Ascidian 
larva with the same in the Amphioxus. Zoologists have thus been 
led to conceive the idea that the Ascidia and the Vertebrata may 
have had a common ancestral origin. 
For an instructive lesson in embryology, I would recommend the 
study of the changes that take place during the development of the 
four-chambered heart of a warm-blooded animal, a bird, a mammal, 
or a man. 
In its earliest form the heart is a simple tube which contracts 
rhythmically upon its contents. It soon becomes constricted in 
some parts and dilated in others, until three chambers are formed, 
analogous to the auricle, ventricle and aortic bulb of a fish, A 
septum now grows in the auricle, dividing it into two parts, the 
completion of which is retarded in the higher animals, owing to the 
conditions of foetal life, but representing when complete the mature 
condition of the heart of the frog and other amphibia. Another 
septum now grows in the ventricle, and the various stages of its 
growth are represented in the permanent forms of the heart in 
various reptiles, until we arrive at the crocodile with the septum 
complete, the lowest animal that possesses a completely four- 
chambered heart. 
Thus the science of embryology forms the key to the compre- 
hension of many of the phenomena of organisation, and is the 
scientific basis of a natural classification of animals. Many abnor- 
malities and congenital malformations can be understood only by 
reference to embryological development ; and the genesis of species 
and the sequence of orders of animals as they have appeared during 
the geologic ages, receive a more rational interpretation when 
viewed by the light of organogenetic development and the relation 
of the various forms of the embryo to the adult. 
It is impossible for me to refer to a tithe of the discoveries that 
have been made in natural science since the introduction of the 
achromatic microscope. ‘The literature of the succeeding period is 
sufficient evidence of their number and value. Dr. Lankester, in 
1860, referring to this literature, said: ‘‘It includes investigations 
with the microscope in every branch of natural science. It contains 
observations on the forms of crystals, plants and animals; it em- 
braces the highest generalisations of physiological science, and 
includes countless investigations into the origin, forms, and modes 
of growth of organs and the ultimate parts of organs of both plants 
and animals. Altogether it forms an assemblage of facts and 
reasonings the most imposing that has ever been presented to the 
human mind in the same space of time in the whole history of 
science,” 
