Page:Popular Science Monthly Volume 67.djvu/122

116 studies, he recognized four types of organization—as Cuvier had done from the standpoint of comparative anatomy. But, since these types of organization have been greatly changed and sub-divided, the importance 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 notochord as occurring in all vertebrate animals.

Von Baer has come to be dignified with the title of the 'Father of modern embryology.' No 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.

In the period between Von Baer and Balfour there were great general 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 establishment of the doctrine of organic evolution.

The Cell Theory.—The generalization that the tissues of all animals 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 imperfect, 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 microscopic structure. Now, for the first time, the tissues of the body were