FEOM EGG TO INSECT — SNODGRASS 385 



FERTILIZATION 



Once the spermatozoon is within the plasm of the eg<^ it travels 

 inAvard, at first straight a^Yay from the point of entrance, though 

 such a course is not necessarily toAvard the egg nucleus. The hitter, 

 as we have seen, is moving at the same time deeper into the egg from 

 the place Avhere it gave off the polar bodies. The sperm soon alters 

 its course to one that will intersect the path of the eg:g nucleus. The 

 two bodies then eventually meet as if mutually draAvn to the same 

 point in the egg plasm, but the influences that effect the meeting are 

 entirely unknoAvn, as are also the forces that bring about the subse- 

 quent activities of fertilization. Consequently we may confine our- 

 selves to a narration of the facts as they occur, which are interesting 

 enough in themselves, Avithout bothering for the present Avith specu- 

 lations as to Avhat causes them. When the sperm and the egg nucleus 

 meet, the sperm casts off its body and tail into the e^g plasm while 

 its nucleated head fuses with the egg nucleus. The substance of the 

 tA\'o nuclei then becomes inclosed in a common membrane, thus estab- 

 lishing a new nucleus, in which the original chromosome number is 

 restored, since this nucleus noAv contains the sum of the female and 

 the male chromosomes. 



The union of the male and female germ cells is the visible phe- 

 nomenon of fertilization, but it brings about such remarkable results 

 in the egg as to shoAv that there are most important processes in- 

 A^olved not j'et rcA'Caled under the microscope. Fertilization accom- 

 plishes at least three things: First, it stimulates the egg to begin 

 dcA^elopment ; second, it adds to the egg a ncAv set of hereditary char- 

 acters or influences deriA'ed from the male line of descent; and third, 

 it apparently determines in the egg of most animals the sex of the 

 future individual. 



The nature of the stimulus given to the egg by the sperm is not 

 i^ertainly known, but that it is a chemical one is indicated by the 

 fact that some eggs can be stimulated to begin deA'elopment by be- 

 ing artifically treated Avith certain substances. Perhaps, on the 

 other hand, the effect is due to the destroying of some inhibiting 

 influence, since insect eggs that develop Avithout fertilization clearly 

 demonstrate that the sperm element is not necessary to them. The 

 hereditary influences brought together by the two conjugating cells 

 constitute a subject too large to be discussed here; the facts asso- 

 ciated with the establishment of sex in the ensuing individual, 

 however, are too interesting to be passed over. 



It is clear that the number of chromosomes in the fertilized egg 

 nucle'us w'ill depend on the number in the particular spermatozoon 



