1M HJSTOLO6 Y 



remainder of the protoplasm lying more centrally, and is less 

 dense, it i- called rr/oy;A/x///. Such a. cell contains no cell mem- 

 brane, and is spoken of as nnlcuL When we find a linn outer 

 boundary for the cell, we call it crusta if there is no definite 

 line of demarcation between it and the contained protoplasm. 

 If, on the contrary, it is sharply marked off on its inner border. 

 we have to do with a true cell membrane. 



The cell membrane may surround the whole cell, in which 

 case it is called pellicula; or it may cover only the free surface 

 of the cell and is then known as 



FIG. 2. 



OtntrotoiHt 



Protoplasmic 

 radiation 



Nuden 



Chroiiiatiit 



Leucocyte from the spleen of Proten* * after Siedlocki'. The centrosome appears in tin- 

 form of two Krannles. The nuclear network is distinct. 



The origin and manner of formation of the cell membrane 

 are not known with certainty ; for it is doubtful whether it is a 

 secretion of the outer layer of the cell, or a modified, hardened 

 part of the protoplasm itself. 



Another cell constituent which has been the object of much 

 attention in the last few years must not be passed over. This is 

 the so-called centrosome. Most authors consider this structure 

 an essential part of the cell (Fio-s. 1 and 2). 



The centrosome occurs usually as one or two granules in the 

 protoplasm, in the neighborhood of the nucleus, and may be 

 contained in u hollowed part of it. Around the centrosome is 

 often to be seen in the protoplasm a radiation which we call the 



