MODERN BIOLOGY 545 



cell-Structure. Indeed, the cellular structure of these lowest organisms has 

 proved to be highly complex, in many of them competing with the funda- 

 mental elements in the highest organisms. Thus there remains nothing to 

 be done beyond widening, by strenuous intensive research, our knowledge 

 of these beings, whose vital manifestations have in many respects proved 

 of service in answering questions of the greatest theoretical interest, not to 

 speak of the important practical problems that have been solved through 

 an extended knowledge of the subject. 



Protozoa 

 Of the unicellular organisms, the Protozoa have, as we know, been longest 

 known; the earlier progress made in this field of research has been described 

 in a previous chapter. Of those who have worked at the subject at a later 

 period the most conspicuous and successful investigators and distinguished 

 teachers were Biitschli and Richard Hertwig. The Protozoa have proved to 

 possess a wealth of different forms and structures, both in protoplasm and 

 nuclei, which has provided the science of general cytology with an invalu- 

 able material for purposes of comparison. Biitschli's investigations into their 

 plasma and the changes that take place therein in different stages have al- 

 ready been mentioned above. Earlier naturalists were inclined to see in the 

 Protozoa radically undifferentiated plasm, but this assumption has been ut- 

 terly disproved by experience; on the contrary, the higher unicellular ani- 

 mals possess a great number of plasmic formations of a markedly organic 

 character — cilia and flagella, vacuoles and muscular fibrillas. And their 

 vital manifestations have after careful investigation been proved to exist in 

 undreamt-of numbers; their irritability and way of reacting to different 

 impressions offer an important field for experimental biology to investigate. 

 The nuclei of the Protozoa have been of special interest owing to their 

 immense wealth of form, to which the higher animal cells have not attained, 

 and owing to their correspondingly numerous functions. It is in this sphere 

 that R. Hertwig has made his most important contribution to the advance- 

 ment of biology. To start with, he has demonstrated that the nucleus in the 

 Protozoa contains the same constituents as the cell-nucleus in general — 

 chromatin, linin, and nuclear body. Moreover, the chromatin substance is 

 in many cases found to be divided up in the cellular plasm in the form of 

 granules or a network, and sometimes the nucleus is entirely incorporated 

 in this latter — a phenomenon that is reminiscent of bacterial chromatin's 

 being invariably distributed over the cellular body, and that at the same 

 time explains part of Haeckel's Monera. Upon the presence of the nucleus 

 depends the Protozoa's capacity for assimilation; a fragment of such a cell 

 without the nucleus would perish for lack of metabolism. Of still greater 

 importance is the condition of the nucleus in the propagation of Protozoa; 

 since this generally takes place through division, the process is started by 



