56 ZOOLOGY DIEECTIOXS 



Draw immature egg. With ocular micrometer find diameter of 

 immature egg and record next to drawing Fig. 6. 



Orientation of the Egg. In Cerebratulus, the polar bodies 

 are given off at the point directly opposite the original point of 

 attachment of the egg in the ovary. The place where the polar 

 bodies are given off is called the ^animal poJe,' while the opposite 

 pole is called the 'vegetative pole.' The line passing from one 

 pole through the nucleus and then through the other pole, is 

 called the 'axis' of the egg. Any line on the egg (or of the 

 embryo) running from pole to pole is called a 'meridional line,' 

 while any plane including a meridian is called a 'meridional 

 plane.' Any plane cutting the axis of the egg (or of the embryo) 

 at right angles is called an 'equatorial plane.' 



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 op the Egg. The process through which 

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

 polar bodies is called 'maturation.' Draw any two different 

 stages. Figs. 7 and 8. 



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 and draw, Fig. 9. 



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. Draw each : 



(a) The cell elongates slightly and begins to constrict, 

 Fig. 10. 



(b) The constriction entirely separates the two daughter 

 cells, leaving but a slight contact surface between them, Fig. 11. 



(c) The two cells become pushed together, forming a broad 

 contact surface one with the other. Fig. 12. 



Observe immature egg and first cleavage stage in same low 

 power field and compare sizes. 



5. Second Cleavage. In succeeding cleavages each cell 



