58 



GENERAL PRINCIPLES OF ZOOLOGY 



nucleoli (true or chromatin-free nucleoli), while the chromatin is dis- 

 tributed on the nuclear reticulum (chromatin reticulum). Much the 

 same may occur in ihe Protozoa (fig. 19, 3). 



Beside and outside of the nucleus there occurs in many Protozoa a 'chromid- 

 ial apparatus, ' a substance agreeing in its staining properties with the nuclear 

 substance. Its pertinence to the nucleus is also shown by the fact that repeatedly 

 it has been observed to arise from the nucleus (ActinosphcFriutn), as well as 

 to be transformed into nuclei (Radiolaria, Monothalamia). The chromidial 

 mass may surround the nucleus like a cortical layer (Euglypha, fig. 20, III, 

 Radiolaria), or penetrate the protoplasm as a loose network (II), or form lumps 

 or coiled threads. In this last shape the chromidial mass seems to be widely 

 distributed in strongly functioning cells of Metazoa (I). Possibly the structures 

 described as ' mitochondria' are identical with it. 



II. 



TIL 



I. 



ch 



FIG. 20. Cells with chromidial apparatus. I, muscle cell of Ascaris (after 

 Goldschmidt). II, Arcella, with two nuclei and loose chromidial net. Ill, Euglypha 

 with compact chromidial envelope of the nucleus, ch, chromidial mass;/, food body; 

 m, mouth of shell; n, nucleus; r, reserve material for new shell. 



Function of the Nucleus. For a long time the function of the 

 nucleus in the cell was shrouded in complete darkness, so that it was 

 regarded, in comparison with the protoplasm, as of little importance. The 

 evidence that the nucleus plays the most prominent role in fertilization 

 has altered this conception. Then arose the view that the nucleus deter- 

 mines the character of the cell ; that the potentiality of the protoplasm is 

 influenced by the nucleus. If from the egg a definite kind of animal 

 develop, if a cell in the animal's body assume a definite histological 

 character, we are, at the present time, inclined to ascribe this to the nu- 

 cleus. From this it follows that the nucleus is also the bearer of heredity; 

 for the transmission of the parental characteristics to the children can 

 only be accomplished through the sexual cells of the parents, the egg and 

 sperm cells. Again, since the character of the sexual cells is determined 

 by the nucleus, the transmission in its ultimate analysis is by the nucleus. 

 This idea has a further support in experiments on Protozoa. If one of 



