EMBRYOLOGY 51 



mal circumstances but a single sperm cell enters each egg cell. 

 Before the union of the nuclei of these germ cells each of the 

 uniting cells has undergone a series of preparatory changes cal.ed 

 Maturation. The fusion of the two germ cells to form a single 

 cell with a single nucleus constitutes the act of fertilization. 

 From the egg thus fertilized a new individual is formed. The 

 early stages in this development will be studied in prepared 

 slides of preserved eggs and embryos. 



You will be given slides in which numerous eggs and em- 

 bryos in various stages of development are mounted in a drop 

 of balsam. THESE MOUNTS ARE VERY FRAGILE AND SHOULD BE 

 HANDLED WITH THE UTMOST CARE. Even in wiping the dust from 

 the coverglass be very careful that no pressure is brought to bear 

 upon the coverglass. 



In this exercise be sure to place the drawings in order on the 

 page. Make six drawings to the page, each one 50 mm. in 

 diameter. Be sure to carry the same relative proportion through- 

 out the entire series of drawings. 



i. THE IMMATURE EGG may be distinguished because of its 

 very large, light colored nucleus. A small, deeply stained spheri- 

 cal body, the nucleolus, is usually conspicuous within the nucleus. 

 Draw immature egg. With ocular micrometer find diameter of 

 immature egg and record next to drawing. 



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 pole' 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 surface of 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! 



