52 LABORATORY DIRECTIONS 



The polar bodies of Cerebratulus always occur inside of the 

 outer membrane of the egg. The egg is always slightly flattened 

 at the point where they appear. 



2. MATURATION OF THE EGG. The process through which 

 the amount of chromatin is reduced by the extrusion of the two 

 polar bodies is called 'maturation/ Make at least two drawings. 



3. FERTILIZATION. After maturation the sperm nucleus, 

 which had entered the egg previously, fuses with the nucleus of 

 the mature egg to produce the first cleavage nucleus. See demon- 

 stration microscope. 



4. FIRST CLEAVAGE. By the process of mitosis the nucleus 

 divides. In the cleavage of the cell which accompanies this in 

 Cerebratulus three processes may be made out; (a) the cell elon- 

 gates slightly and begins to constrict, (b) the constriction en- 

 tirely separates the two daughter cells, leaving but a slight contact 

 surface between them, (c) the two cells become pushed together, 

 forming a broad contact surface one with the other. Illustrate 

 these points in drawings. (At least three drawings.) Observe 

 immature egg and first cleavage stage in same low power field 

 and compare sizes. 



5. SECOND CLEAVAGE. In succeeding cleavages each 

 cell undergoes the same stages outlined in the preceding' para- 

 graph. Make three drawings to show these stages in the second 

 cleavage. 



6. THIRD CLEAVAGE. (A polar and a lateral view.) In 

 this stage note that the four cells at the animal pole are not 

 directly above the four cells of the vegetative pole. This shifting 

 of the position of the cells indicates what is called the 'spiral type 

 of cleavage/ 



7. FOURTH CLEAVAGE. (Polar view.) By focusing note 

 that a cavity is already beginning to form in the center of the 

 mass of cells. In Cerebratulus there is no morula stage. 



8. BLASTULA STAGE. Draw in optical section showing the 

 cavity which is surrounded by. a single layer of cells. . 



