(1) An outermost usually homogeneous non-perforated membrane, which is by most authors regarded as a chorion, but is probably a vitelline membrane--by which name I shall speak of it. (2) A radiately striated membrane (internal to the former when the two coexist) which can be broken up into a series of separate columns. These give to the membrane its radiate striation, but it is probable that between the columns there are pores sufficiently large to admit of the passage of protoplasmic filaments. This membrane will be spoken of as the zona radiata. It is a differentiation of the outermost layer of the yolk. (3) Within the zona radiata a third and delicate membrane is occasionally found, especially when the ovum is approaching maturity. [FIG. 21. SECTION THROUGH A SMALL PART OF THE SURFACE OF AN OVUM OF AN IMMATURE FEMALE OF SCYLLIUM CANICULA. _fe._ follicular epithelium. _vt._ vitelline membrane. _Zn._ zona radiata. _yk._ yolk with protoplasmic network.] In Elasmobranchii the first membrane to be formed is the vitelline membrane, which appears in some instances before the formation of the follicle--a fact which appears to shew that it is really formed as a differentiation of the protoplasm of the egg. In most Elasmobranchii this membrane attains a very considerable development. A zona radiata is generally (if not always) present in Elasmobranchii, but arises at a later period than the vitelline membrane (fig. 21, Zn). The zona radiata always disappears long before the ovum is ripe. The vitelline membrane also gradually atrophies, though it lasts much longer than the zona radiata. When the egg is taken up by the oviduct all trace of both membranes has vanished. In Reptilia precisely the same arrangements of the membranes are found as in Elasmobranchii, except that as a rule the zona radiata is relatively more important. The vitelline membrane is thin except in the Crocodilia. The third innermost membrane is found according to Eimer in many Reptilia. In birds both vitelline membrane and zona radiata are present, but the latter atrophies early, leaving the former as the sole membrane when the egg is ripe.