EMBRYOLOGY OF CEREBRATULUS 



Classification : — Phylum Plathelminthes, Class Nemertinea, 

 Order Heteronemertinea. 



Cerebratiiliis is a marine worm which lives in burrows on 

 sandy beaches. Its reproduction is by the sexual method. When 

 the females reach maturity they discharge enormous numbers of 

 eggs into the surrounding water. At the same time the males 

 discharge myriads of small, motile spermatozoa into the water. 

 These spermatozoa, through their power of locomotion, come 

 into contact with the eggs liberated by the females. Under nor- 

 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 

 called maturation. The fusion of the two germ cells to form a 

 single cell with a single nucleus constitutes the act of fertili- 

 zation. 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 tragile and should be 

 HANDLED WITH UTMOST CARE. Evcu in wiping the dust from 

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

 upon the coverglass. 



Number these drawings in sequence continuous with the 

 drawings of the preceding exercises (Figs. 6-20). 



The drawing of the immature egg should be 50 mm. in 

 diameter and all of the remaining drawings should be in pro- 

 portion. 



1. 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. 



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