EXERCISE VIII 

 EMBRYOLOGY OF TYPICAL ANIMALS 



A. MATURATION AND FERTILIZATION 



The change undergone by the male and female germ cells previous to 

 fertilization is known as maturation. 



1. Maturation of the Male Germ Cells. The undifferentiated male 

 germ cells are known as spermatogonia. These multiply by ordinary mito- 

 sis. When mitosis stops each cell increases in size and is known as a 

 primary spermatocyte. Each primary spermatocyte divides into two 

 secondary spermatocytes. Each of these in turn divides into two sperma- 

 tids which metamorphose into spermatozoa. Thus out of each primary 

 spermatocyte four spermatozoa are formed. During the process of 

 maturation the number of chromosomes is reduced one-half. A wall 

 chart should be studied for the outline of the process. 



la. Examine sections of the testis of the beach grasshopper, T rimer o- 

 tropis maritima. At one end of each section spermatogonia will probably 

 be found, at the other end mature spermatozoa, and between the two ends 

 spermatocytes in various stages. 



16. Note the small size of the spermatogonia. Find some undergoing 

 mitosis. In a polar view of an equatorial plate determine as nearly as 

 possible the number of chromosomes. Record the number. 



Ic. The spermatocytes are larger than the spermatogonia. Among 

 them find cells undergoing mitosis. From an anaphase of the division 

 of a secondary spermatocyte determine as nearly as possible the number 

 of chromosomes. If in doubt whether you are observing the correct 

 stage ask to have one shown to you. How does the number of chromo- 

 somes in each anaphase group in the secondary spermatocyte compare 

 with the number in the spermatogonia? The number in these anaphase 

 groups is the number that goes into the spermatozoa. 



Id. Draw a group of mature spermatozoa, either from the grasshopper 

 or from a mammal, of which a demonstration may be provided. 



2. Maturation and Fertilization of Female Germ Cells. Since, in 

 the animal selected for the study of the female germ cells, the processes 

 of maturation and fertilization occur in large part simultaneously, they 

 are studied together. The chronological order of events is followed. 



2a. Examine sections of the uterus of Ascaris megalocephala. See a 

 specimen of Ascaris. The large rounded bodies are oocytes or later stages. 



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