NATURAL HISTORY NINETEENTH CENTURY. 643 



doubtedly been due to the improvement of the microscope and its 

 appliances. But these have been rendered effective by the discovery 

 of methods of preparing tissues for examination by the use of chemical 

 reagents which harden them, or impart colour, or reveal their structure. 

 Much of what is known of the embryonic development of animals and 

 plants has been the work of the present century. Harvey, Haller, 

 Bonnet, Wolff (1759), and Oken, certainly contributed to found Em- 

 bryology, but it is mainly by the laborious investigations of some dis- 

 tinguished Continental physiologists of the present century that em- 

 bryology has become a great and important department of biology. In 

 1817 CHRISTIAN PANDER had made an elaborate study of the de- 

 velopment of the chick in the egg, and his researches were continued 

 and extended by a man whose name holds the highest place in con- 

 nection with the subject. This was VON BAER, Professor in the Uni- 

 versity of Konigsberg, who, after carrying on his observations for eight 

 years (1819 to 1827), published in 1828 the first volume of his classical 

 work entitled " The Development of Animals." The most remarkable 

 general results of these researches which we shall here mention is the 

 discovery of the universal process of "yolk segmentation" in which the 

 nucleus of the primordial vesicle divides first into two cells, each of 

 these again into two, and so on, the number of cells being successively 

 2, 4, 8, 1 6, 32, etc., until the embryo assumes under the microscope 

 the appearance of a mulberry-like cluster of similar spherical cells en- 

 closed within the original cell-membrane. At a later stage the cells 

 are differentiated in their forms and arrangement. In their first stages 

 all embryos have the same appearance ; a little later the rudiments of 

 the vertebrae are distinguishable in embryos of that class ; but the 

 development must make a considerable progress before the several 

 embryos of mammals, birds, fishes, and reptiles can be discriminated 

 by their structure. 



Large additions to our knowledge of the globe and of its varied 

 forms of life have been made by the expeditions for scientific explora- 

 tion which have been fitted out at the expense of the different Govern- 

 ments, particularly in quite recent times. The deep-sea soundings 

 and dredgings which have so much modified the views of naturalists 

 and geologists, may be said to have been commenced in 1818 by 

 Captain Ross in Baffin's Bay. The voyage of the Challenger, the most 

 recent British enterprise for deep-sea exploration, will be familiar to 

 most readers. The results of these voyages have been in many respects 

 wholly unexpected, and they have especially extended and corrected 

 the views entertained of the condition of the ocean and the distribu- 

 tion of animal life therein. 



Passing over many researches of the highest importance to the 

 avowed student of physiology, we might, had space permitted, have 

 given some account of subjects so likely to command popular interest 

 as the investigations which relate to the nervous system. Such are the 



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