132 PROGRESS OF SCIENCE. 



UNITY OF ORGANIC COMPOSITION the life-matter being 

 " original slime." He also suggested that the development of 

 animal forms proceeds from " mile " cell-growth a profound 

 idea which SCHLEIDEN (1838) and SCHWANN (1839) worked 

 out and fully demonstrated the first tracing the plant embryo 

 to a single nucleus-enclosing cell ; the second tracing the 

 animal embryo to a similar primordial form the cell, there- 

 fore, being the fundamental unit of which every organism, 

 whether plant or animal, is built up. To Schwann may 

 therefore be ascribed the demonstration of the unity of the 

 composition of animal organism. 



1792 1876. Von Baer founded EMBRYOLOGY a very 

 important branch of biology tracing as it does all the stages 

 of development of an animal from its first appearance in the 

 egg to its birth (1828). Von Baer, next, showed a very 

 striking phenomenon, viz. : that the embryos of a fish, a 

 lizard, a bird, an ox are not distinguishable from one another 

 and they continue growing so until a certain stage, and at 

 that stage the fish begins to differ from the three others, and 

 goes on growing into a certain variety of fish ; of the three 

 embryos that remain alike still for a time, one, the lizard, 

 begins to branch off from the common appearance ; then of 

 the two remaining, alike still for a little time, the bird branches 

 off; and lastly the ox embryo takes a decided turn, but only 

 some time after. This great fact offered a new basis for 

 classification, for animals could now be differentiated accord- 

 ing to the special characters they assumed from the beginning, 

 and some defects of classification were rectified. This dis- 

 covery had a philosophical importance besides, for it went far 

 to prove that unity of plan upon which G. St. Hilaire had 

 insisted the embryo showing the same parts developing 

 differently in different animals out of identical beginnings. 

 Von Baer, like Von Buch, anticipated Charles Darwin in 

 believing that a variety may slowly be altered into a per- 

 manent type a doctrine taught long since, as we have seen. 

 In close connection with Von Baer's observations, one of the 

 most instructive additions made to this branch of physiology is 

 due to Mr. Romanes, who, by the use of the microscope again, 

 recognised in the lower class of invertebrates, the jelly-fish 



