14 



THE NEW CELL DOCTRINE 



in appearance and behavior; thus, for example, if we study 

 the development of muscles, we find at first ceUs with the 

 usual so-called undifferentiated protoplasm. In this appear 

 fine fibers which we name fibriUae, and which are no longer 

 simple protoplasm, but really something new, Fig. 9. These 

 fibrils effect the contraction of the muscle. They develop 

 themselves, clearly in order to take over this special function 



of the muscle cells. Accordingly we 

 designate the third condition of pro- 

 toplasm as the differentiated. 



We must now turn to a consider- 

 ation of the nucleus. It appears in 

 the majority of cases as a body with 

 definite limits, completely surrounded 

 by protoplasm and with special sub- 

 stances in its interior. Usually one 

 can distinguish without difl&ctdty a 

 network, and in the meshes of this 

 network the nuclear sap. The net- 

 work varies extraordinarily in the 

 single nucleus, but has one striking 

 peculiarity, namely, that it may be 

 easUy artificially colored. On account 

 of this peculiarity the substance has been named chromatin. 

 Nucleus differs in one respect very noticeably from protoplasm, 

 for the nuclei develop no new structures comparable to those 

 which we may observe in protoplasm. A nucleus, to be sure, 

 changes during the development of tissues more or less, but 

 we cannot observe new structures in the nuclei. This fact is 

 of special significance for the considerations which are to be 

 presented in the next following lecture. For this reason 

 attention is now directed to this peculiarity of the nucleus. 



Fig. 10. — A vesting nucleus 

 after ordinary preservation 

 and staining witii iron hsema- 

 toxyline. From a, cell of the 

 intestinal epithelium of a sala- 

 mander. — Afler M. Heiden- 

 hain. Magnified 2300. 



