12 THE NEW CELL DOCTRINE 



cells, which we find only in the multicellular organisms, that is, 

 in the higher plants and animals. In them we observe different 

 cells which take over the function of propagation. In the case 

 of animals such cells are called ova and spermatozoa. A sper- 

 matozoon unites with an ovum, which we then designate as 

 fertilized. A fertihzed ovum is a complete cell which divides 

 and continues dividing until the number of cells for the con- 

 struction of an animal body has been produced. This number 

 may be enormous. The ovum, or egg-ceU, proUferates by 

 division precisely as does the cell of a unicellular organism. 

 The cells of the latter do not change, but the cells which arise 

 from the ovum do change. The cells of the miilticeUular or- 

 ganisms through several or many early generations retain a 

 relatively similar structure, but later there f oUows a transform- 

 ation which with the succeeding generations progresses, and at 

 the same time becomes multifarious. In this manner the tis- 

 sues of the adult arise gradually and in accordance with fixed 

 laws. 



In consequence of these conditions it has come about that 

 we have derived our conception of protoplasm and in part also 

 of the nucleus chiefly from studies which investigators have 

 made on the developing ova, for in the early generations of 

 these cells we have relatively simple relations. Fortunately, 

 however, there occur among the unicellular organisms species 

 which are comparatively simple in structure, and which are 

 therefore favorable for the study of protoplasm. If we wish 

 to summarize the result of numerous investigations in brief 

 form we may say that we have learned to recognize three 

 conditions of protoplasm; that is, one condition of which 

 we know as yet little, but which is of the greatest significance 

 and which is characterized by the fact that the protoplasm 

 appears to us under the miscroscope absolutely homogeneous. 



