MODERNBIOLOGY 401 



now being dealt with. Especially noteworthy is his investigation into the 

 spermatozoa (1841), in which he proves that they are not parasites, but a 

 true sexual product. Further, his fine monograph Entwkklungsgeschkhte der 

 Cephalopoden (1844), which he worked out in the course of a visit to Naples 

 and which contains an account of egg-division and embryonic development 

 in those animals, to which account subsequent research has had but little 

 to add. Of great importance, too, was his study of the smooth musculature, 

 the elements of which he definitely isolated for the first time, describing 

 them as single-celled fibrillar; their distribution in the different organs of 

 man and the mammals he elucidated with unprecedented completeness. 

 Henle, indeed, had established the musculature of the blood-vessels, but it 

 was Kolliker who explained its character in detail. In the sphere of neurology 

 he also made valuable discoveries; thus, he proved convincingly that the 

 nerve-fibres are connected with processes of the ganglion-cells, thereby 

 making important contributions to the knowledge of their structure. If we 

 add that Kolliker investigated with valuable results certain unicellular 

 animals, as, for instance, the gregarines, we shall have given some idea of 

 his extraordinarily many-sided research work. 



There is one scientist who is worthy of mention by the side of Kolliker 

 — namely, Franz Leydig (1811-1905), who was a native of Wiirttemberg 

 and who was professor at Bonn from 1875 to 1895. As a cytologist he was 

 remarkable for his investigations into the invertebrates. He, too, published 

 a Lehrbuch der Histologic, which, remarkably enough, pays as much attention 

 to the tissues of the invertebrate animals as to those of the vertebrates, 

 thereby laying the foundations of comparative histology, which has since 

 been so extensively developed. 



Leydig's classification of tissues is more in accordance with the modern 

 method than that of Kolliker; thus he groups under the heading "connec- 

 tive substance" not only connective tissue and cartilage, but also bone tissue, 

 which Reichert still kept separate. His presentation of the life and develop- 

 ment of the cell is likewise more modern than Kolliker's, but Leydig's book 

 was published four years later, and during that period cytology made great 

 strides year by year. A good deal of Leydig's own pioneering research-work 

 is recorded in this treatise; his detailed studies of the structure of the insects, 

 especially their digestive, glandular, and sensory organs, should be men- 

 tioned first of all. Leydig's other extremely conscientious microscopical in- 

 vestigations into worms and molluscs, as well as vertebrates, belong to the 

 specialized literature on those subjects; no histological specialist can aff^ord 

 to neglect them, but considerations of space forbid any further reference to 

 them here. 



Cell research entered upon a new phase through the work of Rudolf 

 LuDwiG Carl Virchow. He was born in Pomerania in 182.1, the son of a 



