Schwann. This important principle, propounded in 1838 

 and 1839, produced an immediate effect in unifying or- 

 ganic creatures, though many years passed before it was 

 formulated in the terms employed to-day. In brief, it is 

 this: All the larger organisms are composed of organs 

 which in turn are constructed of various tissues, like muscle 

 and nerve and connective elements; the tissues finally can 

 be resolved into units of structure, the cells, which agree 

 in possessing a central body or nucleus, and in their proto- 

 plasmic substance. The elementary nature of cells is still 

 further demonstrated by the simplest organisms we know, 

 which consist of one cell, nothing more and nothing less; 

 while finally the starting point in the development of 

 higher animals is always a single cell the egg. Truly 

 these are remarkable facts, when we consider the wide 

 range of animal and plant forms. 



Vast as the present knowledge is, the tasks of com- 

 parative anatomy are not entirely completed. Though 

 voyages of exploration like those of the Beagle with Dar- 

 win, the Rattlesnake with Huxley, and above all of the 

 famous Challenger have gone to all parts of the globe, 

 though countless investigators have devoted their lives to 

 the study of special groups like birds and mammals and in- 

 sects and molluscs, every year brings to light new forms 

 that must be analyzed and placed ; while new discoveries in 

 other departments often make it necessary to re-examine 

 known series in the light of fuller knowledge. 



While many naturalists prior to the nineteenth century 

 were interested in the way an animal egg produced an adult 

 organism, it was not until the doctrine of descent energized 

 zoology that comparative embryology attained the inde- 

 pendent status that it holds to this day. Harvey in 1650 

 had perceived that, in his own words, "all animals are in 

 some sort produced from eggs." Bonnet and Haller, of 

 the early eighteenth century, contended that the germ was 



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