LABORATORY OUTLINES FOR 

 EMBRYOLOGY 



CELL DIVISION 



1. All organisms are composed of cells. The essential 

 parts of a cell are: (i) The general cell substance, the 

 cytoplasm, and (2) a highly differentiated nucleus. ]Most 

 cells have a limiting membrane, the cell-wall. Animal 

 cells may have one or more specialized bodies h'ing near 

 or immediately inside the nucleus, the centrosome. The 

 nucleus may be separated from the cytoplasm by a thin 

 membrane, the nuclear membrane. The substance of the 

 nucleus is composed of the chromatin material, which 

 stains readily with certain dyes and the achromatin, 

 which stains less readily. When the cell is in the so- 

 called resting stage there is a portion of the chromatin 

 which is larger, more compact, and stains more densely 

 than the other chromatin granules. This is the chroma- 

 tin nucleolus. There may be more than one nucleolus. 



Draw a cell in the resting stage and label the parts. 



2. When division is about to take place, the chromatin 

 elements in the nucleus have the appearance of a coiled 

 thread, the spireme. The spireme is not always a con- 

 tinuous thread. The nuclear membrane often disappears 

 at this stage. 



