574 BULLETIN OF THE 



changes in the form of the yolk. If the latter diminishes in volume, it can be 

 but little. The vitellus and membrane have a greater diameter than existed 

 before the formation of the membrane. The author therefore speaks of an 

 elevation of the latter, and not of a retraction, which appears to him doubtful. 

 The orifice through the membrane at the " crater " which gave exit to the 

 " cone of attraction," and possibly existed during the early stages of the forma- 

 tion of the " cone of exudation," is no longer to be found after the complete 

 dispersion of the latter cone, nor is the crater longer visible. Directly under- 

 neath this "crater" of the membrane there is a corresponding but smaller 

 depression in the surface of the yolk. This is still visible when the membrane 

 is wholly elevated, but before the male pronucleus is formed. The latter 

 aj)pears as a small clear spot without granules immediately under the vitelline 

 crater. 



The phenomena occurring in the sea-urchin have been considered at page 490. 

 The vitelline membrane is elevated with greater rapidity and energy than in 

 the case of Asterias. The zoosperm suffers little change of form at penetra- 

 tion. It enters progressively by the action of the vitelline sarcode^ and is not 

 impelled by its cue, which has ceased its undulatory movements. The " cone 

 of exudation " is extremely pale and very mobile. The author does not know 

 whether this is a phenomenon of amoeboid contractions or a continuous erup- 

 tion of an almost liquid substance. The body of the spermatozoon once 

 plunged into the yolk is often visible without the aid of reagents. The point 

 of penetration is only determinable by the fact that the female pronucleus 

 retires only part way from the formative pole toward the centre of the yolk. 

 With this as a criterion it may be shown that the penetration takes place at 

 any point, but perhaps more often in the nutritive hemisphere. 



The growth and union of the pronuclei is nearly the same in starfish and 

 sea-urchin. In Asterias the clear spot where the zoosperm penetrated becomes 

 the point of departure for the male pronucleus, which at first remains for sev- 

 eral minutes immovable and without apparent change. The vitelline rays are 

 all directed toward the centre of the spot ; some of them are slightly curved 

 so as to abut at the point of the surface where the cone of exudation still per- 

 sists. The rays become longer and more accentuated with the advance of the 

 aster into the yolk. Its direction, at first centripetal, changes when the female 

 pronucleus does not occupy the centre of the egg, so as to encounter the lat- 

 ter. If the egg is fecundated before the completion of the polar globules, the 

 male pronucleus remains at the edge of the yolk in the condition of a small, 

 hardly visible spot until they are eliminated. Both pronuclei then arise simul- 

 taneously. In this case they meet between the centre and the formative pole, 

 because the male pronucleus advances more rapidly. 



In the sea-urchin, while the clear spot is contiguous to the surface, its in- 

 terior often shows a rounded refringent globule, which appears to correspond 

 to the body of the spermatozoon modified in form, and soon becomes in the 

 living egg invisible. Treatment with osmic acid and carmine shows that the 

 zoosperm preserves a few instants its conical form, then becomes rounded into 



