198 FERTILIZATION OF THE OVUM 



presence of special structures through which the spermatozoon 

 enters (Fig. 100). Thus, the starfish-egg, according to Fol, pos- 

 sesses before fertilization a peculiar protoplasmic " attraction-cone " 

 to which the head of the spermatozoon becomes attached, and through 

 which it enters the egg. In some of the hydromedusae, on the other 

 hand, the entrance point is marked by a funnel-shaped depression at 

 the egg-periphery (Metschnikoff). When no preformed attraction- 

 cone is present, an " entrance-cone " is sometimes formed by a rush 

 of protoplasm toward the point at which the spermatozoon strikes 

 the egg and there forming a conical elevation into which the sperm- 

 head passes. In the sea-urchin (Fig. 100) this structure persists 

 only a short time after the spermatozoon enters, soon assuming a 

 ragged flame-shape and breaking up into slender rays. In some 

 cases the egg remains naked, even after fertilization, as appears to 

 be the case in many coelenterates. More commonly a vitelline mem- 

 brane is quickly formed after contact of the spermatozoon, e.g. 

 in Amphioxus, in the echinoderms, and in many plants, and by 

 means of this the entrance of other spermatozoa is prevented. In 

 eggs surrounded by a membrane before fertilization, the spermato- 

 zoon either bores its way through the membrane at any point, as is 

 probably the case with mammals and Amphibia, or may make its 

 entrance through a micropyle. 



In some forms only one spermatozoon normally enters the ovum, 

 as in echinoderms, mammals, many annelids, etc., while in others 

 several may enter (insects, elasmobranchs, reptiles, the earthworm, 

 Petromyson, etc.). In the former case more than one spermatozoon 

 may accidentally enter (pathological polyspermy), but development 

 is then always abnormal. In such cases each sperm-centrosome 

 gives rise to an amphiaster, and the asters may then unite to form 

 the most complex polyasters, the nodes of which are formed by the 

 centrosomes (Fig. 101). Such eggs either do not divide at all or 

 undergo an irregular multiple cleavage and soon perish. If, how- 

 ever, only two spermatozoa enter, the egg may develop for a time. 

 Thus Driesch has determined the interesting fact, which I have con- 

 firmed, that sea-urchin eggs into which two spermatozoa have acci- 

 dentally entered undergo a double cleavage, dividing into four at the 

 first cleavage, and forming eight instead of four micromeres at the 

 fourth cleavage. Such embryos develop as far as the blastula stage, 

 but never form a gastrula. 1 In cases where several spermatozoa 

 normally enter the egg (physiological polyspermy), only one of the 

 sperm-nuclei normally unites with the egg-nucleus, the supernumer- 

 ary sperm-nuclei either degenerating, or in rare cases e.g. in elas- 

 mobranchs and reptiles living for a time and even dividing to form 







1 For an account of the internal changes, see p. 355. 



