A comparative analysis of the evolution of the egg envelopes and the origin of the yolk

In this paper, the evolution of egg envelope is examined from a structural point of view. The structurally simplest eggs are found in Sponges and Cnidarians where reproduction is ensured by external fertilization. In these instances, the oocyte plasma membrane is protected by only a thin glycocalyx. A more structurally complex envelope is present in triblastic Metazoa where the vitelline membrane makes its first appearance. With the emergence of internal fertilization, the egg envelope is modified as to face the constraints imposed by the terrestrial environment. However the pursuit of such defensive strategy would have made eggs practically impermeable to spermatozoa Eggs with a thick chorion are thus endowed with a proper micropyle to allow fertilization. With the acquisition of viviparity egg envelopes have undergone marked reduction. In this paper particular attention has also been paid to the relationship of the oocyte with accessory cells and to the nature and function of the reserve material stored in the oocyte. The role of cortical granules and of gamete interaction during fertilization is also discussed.