294 GENERAL EMBRYOLOGY 



the formation of the zygotic nucleus from equal contributions 

 of chromosomes from the male and female parents. This is 

 the only portion of the new organism which is derived equally 

 from both parents, and while it is true that a small amount of 

 cytoplasm accompanies the sperm nucleus in its entrance into 

 the ovum, this varies considerably in amount in different forms. 

 This latter fact together with the general fact of the primary 

 importance of the nucleus in all aspects of cell activity, com- 

 bine to enhance the significance of the equal derivation of the 

 chromosomes (Fig. 106), especially in view of the further fact 

 that on the whole the family and individual traits of organisms 

 are inherited with equal likelihood from either parent. 



The behavior of the chromosomes throughout the matura- 

 tion process affords many highly interesting and significant 

 parallels between chromosome behavior and the facts of hered- 

 ity. Interest centers here in the phenomena of synapsis 

 and the " reducing" divisions. 



Any precise interpretation of these two phenomena seems 

 impossible until more is known with certainty regarding the 

 behavior here of the chromatin granules, but the phenomena 

 themselves are readily interpretable in the light of the facts of 

 alternative, or Mendelian heredity. 



In synapsis we see the final union of pairs of chromosomes 

 introduced into a single nucleus at the time of fertilization, but 

 remaining distinct throughout the life of the hybrid generation, 

 until the time when the hybrid organism forms its gametes. 

 Synapsis is not a haphazard junction of chromosomes, but an 

 orderly union of elements of paternal and maternal origin, 

 similar in size, in details of form, and probably also in function. 

 The bivalent chromosomes thus formed are, in consequence of 

 their derivation from two individuals, not quite homogeneous 

 throughout. Following synapsis come two divisions of each 

 chromosome, and in most organisms one of these apparently 

 divides the chromosome equally, into two similar parts (equa- 

 tion division), while the other divides each of the daughter 

 chromosomes dissimilarly (reducing division), the dissimilarity 

 resulting from the relation of the plane of division to the plan 



