46 



SCIENCE 



[N. S. Vol. XLIII. No. 109S 



bach' described the appearance of two nu- 

 clei at opposite ends of the elongated egg 

 of Rhabdites; these increase in size, mi- 

 grate towards the center of the egg, meet, 

 rotate through 90° and fuse together. A 

 dicentric figure appears and cleavage fol- 

 lows. What is the origin of these two nu- 

 clei and the significance of their union? 

 The fusion of two nuclei was at the time 

 entirely without analogy. Auerbach states : 



It is natural to assume that, as for tlie reproduc- 

 tion of organisms the copulation of two individ- 

 uals, or at least of two cells in some form or other 

 is so frequently necessary, so here a similar condi- 

 tion is found for nuclear reproduction. 



Auerbach supposes the two nuclei which 

 appear at opposite ends of the elongated 

 egg to have arisen freely ; one of these comes 

 from the end where the spermatozoa had 

 penetrated, the other from the opposite end 

 where the germinal vesicle had disappeared. 

 The difference of the origin influences the 

 quality of the nuclear materials arising de 

 novo; fusion of the nuclei counteracts the 

 differences thus arising ; but aU this would 

 be undone if the division of the fusion nu- 

 cleus followed along the plane of the union ; 

 hence the rotation through 90°. 



In the next year Biitschli again observed 

 fusion of nuclei in nematode eggs before 

 the first cleavage. However, he did not ac- 

 cept Auerbach 's interpretation, but he 

 tended to regard it as a general law of nu- 

 clear formation, that first two or several 

 small nuclei arise and subsequently fuse; 

 this he finds to occur even in the blast- 

 omeres of the four- and eight-cell stages. 



About the same time (1875) Van Bene- 

 den also observed similar phenomena in the 

 rabbit's egg. He did not see spermatozoa 

 enter the egg, but he found them with their 

 heads closely applied to the surface in every 

 unsegmented egg, and came to the conclu- 

 sion that fertilization consisted essentially 



s ' ' Organologische Studien. ' ' 



in fusion of the spermatic substance with 

 the superficial layer of the vitellus. At a 

 little later stage he found a small nucleus 

 in the cortical layer of the egg; this he 

 called the peripheral pronucleus; a central 

 pronucleus appeared simultaneously. They 

 grow, approach one another and meet in 

 the center. Later there is only one nucleus, 

 probably formed by the union of the two. 



As I have shown that the spermatozoa attach to 

 the surface of the vitellus and mix with its super- 

 ficial layer, it appears probable to me that the 

 superficial pronucleus is formed, partially at least, 

 at the expense of the spermatic substance. If, as 

 I think, the central pronucleus is constituted of 

 elements furnished by the egg, the first nucleus of 

 the embryo would be the result of union of male 

 and female elements. I put forth this latter idea 

 simply as a hypothesis, an interpretation which 

 may or may not be accepted. 



The way was now clear for the definitive 

 solution of the old riddle of the relation of 

 the egg and spermatozoon, which was 

 quickly furnished by 0. Hertwig and Her- 

 mann Fol. The observations of these au- 

 thors appear to have been made independ- 

 ently and nearly simultaneously. In 1875 

 Hertwig observed and described correctly 

 the principal phenomena of fertilization in 

 the sea-urchin egg. He did not actually 

 see the penetration of the spermatozoon, 

 but he observed the sperm nucleus and its 

 aster so soon after that he had no doubt of 

 the correct interpretation; he also observed 

 the approach of the sperm-nucleus and the 

 egg-nucleus to the center of the egg and 

 their apparent fusion. 



Fertilization has been previously interpreted as 

 a fusion of two cells, but we have now seen that 

 the most important process involved is the fusion 

 of the two nuclei. The union of the egg-nucleua 

 with the sperm-nucleus is necessary to produce a 

 nucleus endowed with living forces adequate effec- 

 tively to stimulate the later developmental proc- 

 esses in the yolk, and to control them in many re- 

 spects. 



Fol's observations, made partly independ- 

 ently of Hertwig 's and partly after the 



