[27] EMBRYOGRAPHY OF OSSEOUS FISHES. 481 



prevents the entrance of other spermatozoa. In the three above-mentioned 

 genera of osseous fishes more than one spermatozoon cannot therefore enter 

 the egg. 



"Immediately below the inner opening of the micropyle, and at the 

 point where the spermatozoon entered the germ and before the spindle 

 has completely disappeared, there appears, although extremely small, 

 yet plainly visible, a new aster or radial fignre, and in its clear center 

 a second small nncleus is developed — the male pronucleus. Around 

 both nuclei the protoplasmic granules are arranged in sharply defined 

 rays. Both nuclei then become gradually larger and larger, approach 

 each other, to finally blend together to form the first segmentation nu- 

 cleus. Before this conjugation takes place the germ has been completely 

 aggregated by contractile movements of its substance at the micropylar 

 pole of the eg^. The eggs of Scorpwna and Julis only are adapted to 

 the study of the phenomena here described. In the eggs of Scorpccna a 

 very inconsiderable space is formed around the virellus, between the 

 latter and the yelk, which becomes noticeable only when the egg is get- 

 ting ready to segment. The same remark applies to the eggs o£ Julis. 

 In Crenilabrus the water space is, on the other hand, more spacious, but 

 in that it is developed very late in the neighborhood of the inner micropy- 

 lar opening, the germ remains in close contact with the inner openiug of 

 this canal, so that in these three genera of osseous fishes the polar body 

 can be pushed out only through the canal, in that there is no space 

 between the germ and the zona radiata. In other cases, as soon as the 

 spermatozoon has come in contact with the germ a large paravitelline 

 cavity or water space is formed, as in Heliasis, for example. In conse- 

 quence of this, the polar body which is extruded cannot be thrust out 

 into the micropylar canal, but remains within the paravitelline space. 

 Since, in the cases of Scorpcena, Juli^, and Crenilabrus, but one sperma- 

 tozoon can enter the egg, it is highly i^robable, that the same is true of 

 the ova of all osseous fishes, although it may not be jjossible at this mo- 

 ment to say with certainty at what time the entrance of other sperma- 

 tozoa is interrupted. In those cases also in which a large water space is 

 formed between the germ and micropyle, perhaps the tension of the 

 zona radiata in such instances is effectual in closing the inner opening 

 of the micropyle which ends on a papilliform internal prominence of the 

 zona. In the water space within the zona I never saw any spermatozoa. 

 The spermatozoa can enter the egg only through the microjiyle. The 

 phenomena which manifest themselves in mature eggs when simply 

 placed in water without spermatozoa are very variable. Of one and the 

 same lot of eggs of which a part were fertilized and developed regularly, 

 the other part, after lying in water for twenty-four hours, did not show the 

 slighest alteration; in others again, after four hours, the spindle had dis- 

 appeared, the polar body was extruded, and the germ was as well de- 

 veloped as if the egg had been fertilized, with only this diflereuce, that 

 the germ was formed much more slowly than in the fertilized egg-, iu 

 S. Mis. 46 31 



