28 TEXT-BOOK OF EMBRYOLOGY. 



division each dyad gives one of its pieces to each daughter cell (mature ovum, 

 second polar body, spermatid) (Figs. 9 and 14). 



In reduction without tetrad formation, the spireme segments into the re- 

 duced number of pieces. Each of these segments, however, does not show an 

 immediate differentiation into four pieces to form a tetrad, as in the case of 

 reduction with tetrad formation; but in the first maturation division each 

 chromatin segment divides equally, giving half its substance to each daughter 

 cell. Then there may be a more or less complete reconstruction of the nucleus. 

 Finally, in the second maturation division, each of the halves of the original 

 chromatin segments divides again, one part passing to each daughter cell. 



Comparison of the two types of reduction makes evident the fact that there 

 is no essential difference between them, other than the time of division of the 

 reduced number of chromatin masses into their ultimate number for distribu- 

 tion to the four granddaughter cells. In reduction with tetrad formation, the 

 four ultimate divisions are evident from the first. In reduction without tetrad 

 formation no such subdivision is apparent, the division of each chromatin mass 

 into two being accomplished apparently by the first maturation division and 

 into the final four of the mature cells, by the second maturation division. 



The apparent difference between male and female maturation — the single 

 functional cell in the female as contrasted with the four in the male — loses some 

 of its significance when one notes that in some forms the polar bodies are not 

 so rudimentary as is generally the case. Thus in certain forms one or more of 

 the polar bodies may develop into cells very similar to the mature egg cell, may 

 be penetrated by spermatozoa, and may even become fertilized and proceed a 

 short distance in segmentation. There is thus warrant for considering the 

 polar bodies rudimentary or abortive ova. 



Of interest in connection with the apparent necessity, in sexual reproduction, 

 that the egg rid itself of the polar bodies before the female pronucleus is ready 

 for union with the male pronucleus, is the fact that in certain parthenogenetic 

 eggs only a single polar body is extruded, while the eggs of the same species 

 which are to be fertilized produce two polar bodies; also that in certain cases of 

 parthenogenesis the second polar body has been shown to form what is ap- 

 parently a pronucleus, so that there are two pronuclei within the egg cytoplasm, 

 both derived from the secondary oocyte. These unite to form the segmenta- 

 tion nucleus, the second polar body thus acting the part of the male pronucleus. 



A brief discussion of accessory chromosomes will be found under the 

 head "Determination of Sex" in the chapter (XV) on the Urogenital System. 



Theoretical Aspects of Reduction. 



When we come to consider why reduction of chromosomes occurs, we are led into a. 

 maze of conflicting hypotheses. The field is purely speculative in character. Some of 



