BEHAVIOR OF THE GAMETES 241 



blastula. This apparent independence of the early cleavage blastomeres in the 

 opossum and their lack of cohesiveness is evident in other mammals, also. 

 The tendency of the blastomeres in mammals in general to separate from 

 each other emphasizes the importance of the zona as a capsule which func- 

 tions to hold the blastomeres together. 



The surrounding egg membrane, in many cases, may act osmotically to 

 permit a nice balance between the developing egg and the substances outside 

 of the membrane. For example, in birds, the egg and its contained embryo 

 together with its immediate environment are largely maintained as a physico- 

 chemical system due to the osmotic properties of the zona radiata or vitelline 

 membrane. This membrane separates the watery albumen from the nutritive 

 yolk material. These two substances have different osmotic conditions. Con- 

 sequently, the vitelline membrane must maintain the proper conditions be- 

 tween these two general areas, and it performs this function in an admirable 

 fashion. It should be emphasized further that the viteUine membrane in the 

 chick's egg is a living membrane, and consequently its osmotic properties are 

 different from that of a non-living membrane, such as a collodion membrane. 

 If the egg and albumen of the hen's egg are separated by a thin collodion 

 membrane, for example, they will reach an osmotic equilibrium more rapidly 

 than when separated by the thin vitelline membrane. If, however, the vitelline 

 membrane is isolated from its normal relationships in the egg, it behaves 

 similarly to a collodion membrane. It is best to regard the vitelline membrane, 

 the yolk, and the albumen of the bird's egg as forming an harmonious system, 

 in which all parts are responsible for the maintenance of the necessary condi- 

 tions for development. (Consult Romanoff and Romanoff, '49, pp. 388-391.) 



Undoubtedly in other eggs, such as that of the frog, the delicate relation- 

 ship existing between the egg, the perivitelline fluid, the vitelline membrane, 

 and the surrounding external medium forms a complete unit for the proper 

 maintenance of developmental conditions. In most eggs the vitelline or similar 

 membranes maintain the protective function until a relatively late period in 

 development. 



b. Fertilization Cone or Attraction Cone 



The fertilization cone results from specialized activity of the surface of the 

 egg (egg cortex) at the point of sperm contact (fig. 130). This structure has 

 been described in various invertebrate eggs, such as those of the sea urchins, 

 annelid worms, mollusks, and in some of the ascidians among the proto- 

 chordata. In the annelid worm. Nereis virens, as the sperm makes its way 

 through the egg membrane, a cone of cortical ooplasm flows out to meet the 

 sperm, making an intimate contact with the perforatorium (acrosome) of the 

 sperm (fig. 130B, C). When this contact is made, the extended cone with- 

 draws again gradually, and appears to pull the sperm head into the egg's 

 substance (fig. 130D-G). In the egg of the sea urchin, Toxopneustes varie- 



