ix6 POPULAR SCIENCE MONTHLY. 



studies, he recognized four types of organization — as Cuvier had done 

 from the standpoint of comparative anatomy. But, since these types 

 of organization have heen greatly changed and sub-divided, the impor- 

 tance of the distinction has faded away. But as a distinct break with 

 the old idea of a linear scale of being it was of moment. 



Among his especially noteworthy discoveries may be mentioned 

 that of the egg of the human being and other mammals, and the noto- 

 chord as occurring in all vertebrate animals. 



Von Baer has come to be dignified with the title of the ' Father of 

 modern embryology.' ISTo man could have done more in his period, 

 and it is owing to his superb intellect, and talents as an observer, that 

 he accomplished what he did. As Minot says : He ' worked out, almost 

 as fully as was possible at this time, the genesis of all the principal 

 organs from the germ-layers, instinctively getting at the truth as only 

 a great genius could have done.' 



After his masterly work the science of embryology could never 

 return to its former level ; he had given it a new direction, and through 

 his influence a period of great activity was inaugurated. 



The Period from Von Baer to Balfour. 



In the period between Von Baer and Balfour there were great gen- 

 eral advances in the knowledge of organic structure which brought 

 the whole process of development into a new light. 



Among the most important advances are to be enumerated: the 

 announcement of the cell theory, the discovery of protoplasm, the 

 beginning of the recognition of germinal continuity and the establish- 

 ment of the doctrine of organic evolution. 



The Cell Theory. — The generalization that the tissues of all ani- 

 mals and plants are structurally composed of similar units — called 

 cells — was given to the world through the combined labors of Schleiden 

 and Schwann. Schleiden, the botanist, in 1838, and Schwann, the 

 anatomist, in the following year, published the observations on which 

 this truth rests. The investigations stimulated by the announcement 

 of this theory soon resulted in showing that the conception of the cell 

 entertained by the founders was very iniperfect, and, by 1860, the 

 original theory had been molded into the protoplasm doctrine of Max 

 Schultze. 



The modification of the cell theory did not, however, affect the 

 original conception that the cell is a unit of organic structure, but 

 showed that the unit is, essentially, a globule of protoplasm containing 

 a nucleus, and not simply a box-like compartment as Schleiden and 

 Schwann had suggested. 



The broad-reaching effects of the cell-theory may be easily imagined 

 since it united all animals on the broad plane of similitude in micro- 

 scopic structure. Now, for the first time, the tissues of the body were 



