172 GENERAL EMBRYOLOGY 



And once within the ovum, a " foreign " sperm seems to act 

 almost as efficiently as the proper sperm in inaugurating 

 development. After the entrance of a foreign sperm the two 

 germ nuclei may not, usually do not, fuse, and other internal 

 developmental processes may not be entirely normal, but the 

 external processes of cleavage and differentiation may proceed 

 normally for some time, even to the formation of a free-swrniming 

 larva, as in many species of Echinoderm eggs fertilized by the 

 sperm of other species, genera, or even of other classes of Echino- 

 derms (Baltzer), or of other phyla (Mollusca, Kupelwieser). 



In many species the entire spermatozoon enters the cyto- 

 plasm of the ovum (some of the Turbellaria, Annelids, Insects, 

 Molluscs, and many Vertebrates) while in others the tail piece 

 separates from the remainder of the sperm cell and is left 

 outside of, or embedded within, the vitelline membrane, so 

 that only the head and middle piece actually share in the 

 formation of the zygote. In some instances the middle piece, 

 too, fails to enter the egg (e.g., Nereis, Fig. 86). Once within 

 the egg, the sperm continues its inward course for a short 

 distance only. If the entire sperm cell has entered, one of the 

 first events is a sharp bending or flexure between the tail and 

 middle piece, often followed by a separation of the two, after 

 which the tail piece is left behind as the remainder continues 

 its migration (Fig. 88). 



The entrance of the spermatozoon within the egg cytoplasm 

 is the event which inaugurates a whole series of fertilization 

 processes culminating in the formation of a typical mitotic 

 division figure within the zygote. The precise character of the 

 stimuli which start this chain -of actions is still in doubt, but it 

 seems likely that in many instances it acts, first, by bringing 

 about the formation of a permeable membrane over the surface 

 of the egg, through which may occur rapid and extensive 

 osmotic interchanges leading to marked oxidations; and second, 

 by reducing the amount of fluid in the egg cytoplasm, either 

 actually, by its loss through the permeable membrane, or 

 relatively by the addition of the much denser substance of the 

 spermatozoon itself (Loeb). At any rate, whether it be a 



