126 FUNDAMENTALS OF CYTOLOGY 



and carrying out the first embryonal mitosis. In other cases both pro- 

 nuclei have passed independently through most or all of the prophasic 

 stages before meeting; hence, when their membranes disappear their 

 two genomes lie together in the achromatic figure of the first embryonal 

 mitosis, and they first become enclosed in a common nuclear membrane 

 at the telophase of this mitosis. This more or less independent behavior 

 of the genomes w^th no intimate nuclear union before the first embryonal 

 mitosis is known as gonomery. In some organisms the two genomes 

 continue to act as two visibly distinct groups of chromosomes through 

 several embryonal mitoses. 



This close association of syngamy with the first embryonal mitosis is 

 emphasized further by the behavior of the centrioles. The origin of 

 the centrioles seen at the poles of the achromatic figure in the first 

 embryonal mitosis is not altogether clear, but the characteristic appear- 

 ance of an aster with a centriole in the cytoplasm near the base of the 

 sperm head is significant. Either the centriole is that known to have 

 been incorporated in the sperm during spermiogenesis, or the sperm 

 in some way induces the formation of an aster and centriole by the egg 

 cytoplasm. It has been shown that by treating echinoderm eggs with 

 certain chlorides the formation of numerous asters can be induced in the 

 cytoplasm and that two of these, especially if they originate near the 

 nucleus, can function in mitosis. Whatever the origin of the aster in 

 normal syngamy, it divides to two which occupy the poles of the first 

 embryonal mitotic figure. The entrance of the sperm is not merely a 

 necessary preliminary to syngamic nuclear union; it also affects the 

 processes leading to the division of the cell. 



Syngamy has two effects of cardinal importance. The first of these is 

 activation, by which certain physiological processes are set in motion or 

 greatly accelerated. In most cases this leads to the immediate develop- 

 ment of the cell into a new individual, but in some animals and plants 

 the cell develops certain protective coats and enters a dormant state 

 from which it emerges later under the appropriate environmental condi- 

 tions. In cither event there is a profound physiological change at the 

 time of syngamy. This is not dependent upon the union of the gametic 

 nuclei, for not only are some of the results of the change manifested long 

 before the union occurs, but complete activation maj^ be induced by 

 various physical and chemical treatments in the absence of sperms. 

 Furthermore, some animal eggs are naturally parthenogenetic, undergoing 

 complete development without syngamy regularly in successive life 

 cycles. The egg has accordingly been termed an inde'pendently activahle 

 system which contains everything necessary to development, even 

 though a stimulus of one kind or another is ordinarily required to initiate 

 its fiu'ther activity. 



