162 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



Many vertebrate and protochordate eggs possess a bilateral symmetry 

 which becomes evident when the fertilization processes are under way or 

 shortly after their conclusion. The appearance of the gray crescent in the 

 frog's egg (fig. 11 9K) and in other amphibian eggs during fertilization and 

 the similar appearance of the yellow crescent in the fertilized egg of the 

 ascidian, Styela (fig. 132D) serve to orient the future right and left halves 

 of the embryo. Conditions similar to that of Styela, but lacking the yellow 

 pigment, are present in Amphioxus. Similarly, in the chick, if one holds the 

 blunt end of the egg to the left, and the pointed end to the right, the early 

 embryo appears most often at right angles, or nearly so, to the axis extending 

 from the broader to the smaller end of the egg, and in the majority of cases 

 the cephalic end of the embryo will appear toward the side away from the 

 body of the observer. There is some evidence that the "yolk" or egg proper 

 is slightly elongated in this axis. It appears, therefore, that the general plane 

 of bilateral symmetry is well established in the early chick blastoderm, although 

 the early cleavages do not occur in a manner to indicate or coincide with 

 this plane. In prototherian mammals, a bilateral symmetry and an antero- 

 posterior orientation is established in the germinal disc at the time of ferti- 

 lization, soon after the second polar body is discharged (fig. 136). 



5) Membranes Developed in Relation to the Oocyte; Their Possible 

 Sources of Origin. A series of membranes associated with the surface of 

 the oocyte are formed during its development within the ovary. Three general 

 types of such membranes are elaborated which separate from the oocyte's 

 surface at or before fertilization, leaving a perivitelline space between the 

 egg's surface and the membrane. They are: 



( 1 ) A true vitelline membrane which probably represents a specialization 

 or product of the ooplasmic surface. For a time this membrane adheres 

 closely to the outer boundary of the ooplasm, but at fertilization it 

 separates from the surface as a distinct membrane. 



(2) A second membrane in certain chordates is elaborated by the follicle 

 cells. It is known as a chorion in lower Chordata but is called the 

 zona pellucida in mammals. 



(3) A zona radiata or a thickened, rather complex, membrane is formed 

 in many vertebrates; it may be considered to be a product of the 

 ooplasm or of the ooplasm and the surrounding follicle cells. 



All of the above membranes serve to enclose the egg during the early 

 phases of embryonic development and therefore may be considered as pri- 

 mary embryonic membranes. As such, they should be regarded as a definite 

 part of the egg and of the egg's differentiation in the ovary. A description 

 of these membranes in relation to the egg and possible source of their origin 

 in the various chordate groups is given below. 



a) Chorion in Styela. A previously held view maintained that the chorion 



