Human Physiology. 261 



are formed of matter already fitted for such structures. These 

 cells undergo rapid transformations, and become the blood, 

 muscle, bone, all the tissues of the young animal. In the egg, 

 these cells are seen to become more opaque in some parts, 

 more transparent in others ; they divide and subdivide, until 

 the yolk forms what is called a mulberry mass. A germ gathers 

 upon the surface, and separates into three layers. In the eggs 

 of fishes, which are so transparent as to be easily watched 

 through the process of development, may be seen an upper or 

 nervous layer, in which are formed the organs of animal life 

 bones, muscles, brain and nerves, &c. The lower layer gives 

 origin to the organs of vegetative life the abdominal viscera, 

 intestines, or alimentary system, the intermediate layer pro- 

 duces the heart, arteries, veins, &c., of the system of circulation. 



At a very early period, the general form of the insect or 

 animal is manifested. In insects and crustaceans, the germ is 

 divided into sections. In the germs of vertebrate animals, there 

 are seen the rudiments of a spinal canal, which, when formed, is 

 filled with a fluid, from which is formed the brain and spinal 

 cord. The embryo rests upon the yolk, and covers it like a 

 cap, vertebrates enclosing it by the edges uniting at the navel. 



In fishes, whose embryonic development has been carefully 

 observed by Professor Agassiz, the first lines of the embryo 

 appear on the tenth day a canal, which becomes a tube the 

 spine, and an enlargement at one end, the rudimentary head, 

 in which may soon be seen a division of the brain for the 

 organs of sight, hearing, and smell ; and soon after the rudi- 

 ments of eye and ear are apparent. About the seventeenth day 

 the heart is seen as a simple cavity, and, as soon as it is closed, 

 there are regular contractions and a movement of blood cor- 

 puscles. On the thirtieth day there is a regular circula- 

 tion of blood ; the tail, gets free, and moves in violent jerks, 

 and the head is soon liberated. The fish has a brain, an intes- 

 tine, a pulsating heart, and a limited amount of spontaneous 



