GENERAl. CONSIDERATIONS 113 



d) Mitochondrial material and formation of the middle piece of the sperm 



e) The cytoplasm, axial filament, mitochondria, and tail formation 

 c. Cytoplasmic differentiation of the egg 



1 ) Types of chordate eggs 



a) Homolecithal (isolecithal) eggs 



b) Telolecithal eggs 



2) Formation of the deutoplasm 



3) Invisible morphogenetic organization within the cytoplasm of the egg 



4) Polarity of the egg and its relation to body organization and bilateral sym- 

 metry of the mature egg 



5) Membranes developed in relation to the oocyte; their possible sources of 

 origin 



a) Chorion in Stye la 



b) Egg membranes of Ampltioxiis 



c) Vitelline membrane and zona radiata of elasmobranch fishes 



d) Zona radiata of teleost fishes 



e) Vitelline membrane (zona radiata) in amphibia 



f) Zona radiata (zona pellucida) of the reptile oocyte 



g) Vitelline membrane (zona radiata) of the hen's egg 

 h) Membranes of the mammalian oocyte 



5. Physiological maturation of the gametes 



a. Physiological difi'erentiation of the sperm 



b. Physiological ripening of the female gamete 

 D. Summary of egg and sperm development 



A. General Considerations 



In the two preceding chapters the conditions which prepare the male and 

 female parents for their reproductive responsibilities are considered. This 

 chapter is devoted to changes which the male and female germ cells must 

 experience to enable them to take part in the processes involved in the repro- 

 duction of a new individual. 



The gamete is a highly specialized sex cell or protoplasmic entity so dif- 

 ferentiated that it is capable of union (fertilization; syngamy) with a sex cell 

 of the opposite sex to form the zygote from which the new individual arises. 

 The process of differentiation whereby the primitive germ cell is converted 

 into the mature gamete is called the maturation of the germ cell. 



The main events which culminate in the fully-developed germ cell are 

 possible only after the primitive or undifferentiated germ cell has reached a 

 certain condition known as the definitive state. When this stage is reached, 

 the germ cell has acquired the requisite qualities which make it possible for 

 it to differentiate into a mature gamete. Before the definitive state is reached, 

 germ cells pass through an eventful history which involves: 



( 1 ) their so-called "origin" or first detectable appearance among the other 

 cells of the developing body, and 



(2) their migration to the site of the future ovary or testis. 



After entering the developing substance of the sex gland, the primitive 

 germ cells experience a period of multiplication. If the sex gland is that of 



