VON BAER AND RISE OF EMBRYOLOGY. 117 



analyzed into their units; now, for the first time, was comprehended 

 the nature of the germ-layers of Von Baer. 



Among the first questions to emerge in the light of the new re- 

 searches were : What is the origin of the cells in the organs, the tissues 

 and the germ-layers ? The road to the investigation of these questions 

 was already opened, and it was followed, step by step, until the egg and 

 sperm came to be recognized as modified cells. This position was 

 reached, for the egg, about 1861, when Gegenbaur showed that the eggs 

 of all vertebrated animals, regardless of size and condition, are in 

 reality single cells. The sperm was put in the same category about 

 1865. 



The rest was relatively easy— the egg, a single cell — by successive 

 divisions produces many cells, and the arrangement of these into 

 primary embryonic layers brings us to the starting point of Wolff and 

 Von Baer. The cells, continuing to multiply by division, not only 

 increase in number, but also undergo changes through division ! of 

 physiological labor, whereby certain groups are set apart to perform a 

 particular part of the work of the body. In this way arise the various 

 tissues of the body — which are, in reality, similar cells performing 

 a similar function. Finally, from combinations of tissues the organs 

 are formed. 



But the egg, before entering on the process of development, miist 

 be stimulated by the union of the sperm with the nucleus of the egg, 

 and, thus, the starting point of every animal and plant, above the 

 lowest group, proves to be a single cell with protoplasm derived from 

 two parents. While questions regarding the origin of cells in the 

 body were being answered, the foundation for the embryological study 

 of heredity was also laid. 



Advances were now more rapid and more sure, flashes of morpho- 

 logical insight began to illuminate the way, and the facts of isolated 

 observations began to fit into a harmonized whole. 



Apart from the general advances of this period, mentioned in other 

 connections, the work of a few individuals requires notice. 



Bathke and Remak were engaged with the broader aspects of em- 

 bryology as well as with special investigations. To Rathke is owing 

 great advances in the knowledge of the derelopment of insects and 

 other invertebrates, and Bemak is notable for similar work with the 

 vertebrates. As already mentioned, he was the first to recognize the 

 middle layer as a unit — through which the three germ-layers of later 

 embryologists emerged into the literature. 



Koelliker, the veteran embryologist, still living in Wiirzburg, car- 

 ried on investigations on the segmentation of the egg. Besides work 

 on the invertebrates, later, he followed with care the development of 

 the chick and the rabbit — he encompassed the whole field of embryol- 

 ogy—and published, in 1861 and later, in 1876, a general treatise on 



