728 LIFE : OUTLINES OF GENERAL BIOLOGY 



(or hereditary factors, or developmental initiatives), the maternal 

 and paternal contributions being as regards essentials duplicates. 

 But if in a paternal chromosome there was a peculiar gene, a novel 

 Mendelian difference, which might be the initiative of musical genius 

 or of red hair, there is an obvious chance that it might be lost in 

 the first polar body. And if the spermatozoon did not happen to 

 bring in that particular gene from the male (we are now passing 

 to a new paternal contribution, not of the father, but of the mate), 

 then the peculiarity in question would be lost from the inheritance 

 for the time being .And the same would be true of a minus feature, 

 such as some pathological taint. 



As the facts are intricate, let us repeat ourselves in reference to 

 a maternal chromosome in the unripe egg. It contains the genes of 

 a group of hereditary initiatives which are characteristic of the 

 species, but it may also contain a peculiarity that was expressed in 

 the mother of the organism whose germ-cells we are discussing. 

 The peculiarity might be unusual fecundity or it might be crinkly 

 hair ; and its gene might be lost in the first polar body. Yet it might 

 be re-introduced into the fertilised ovum by a spermatozoon coming 

 from a mate in whom the gene for fecundity or for crinkly hair was 

 carried by all or by half of the final spermatocytes. Moreover, a 

 particular novel gene in one of the chromosomes of the reduced 

 female pronucleus may be supplemented by a similar novel gene 

 carried by the fertilising spermatozoon. This would be what the 

 Mendelians call a "double dose" of that particular quality; and when 

 the quality is on the minus side the "double dose" may have serious 

 consequences. Yet it must be remembered that in most cases there 

 are many ova and many spermatozoa, differing in the details of 

 their maturation. This is part of the reason for the familiar specific 

 inertia — the persistent begetting of like by like. ' 



The biological significance of the maturation division is partly 

 that in the meiosis a chromosome separated off may carry away a 

 gene that is not restored in fertilisation. This tends to variability, 

 and it enables us to understand Mendelian inheritance. Moreover, 

 in the intimate contact of a pair of chromosomes in synapsis there 

 may be an exchange of pieces of the two chromosomes. A portion 

 of chromosome A may slip over into chromosome A', and vice 

 versa. This "crossing over", as it is called, is another possibiHty 

 in the shuffling of the hereditary cards; and it makes for variation. 



In ordinary mitotic division, which has been previously described, 

 each chromosome is longitudinally split down the middle, and each 

 daughter- cell receives a meticulously accurate half of everything in 

 the cell, including a longitudinal half of each chromosome. So the 

 normal number n persists. In meiosis, as has been explained, the 

 number of chromosomes is halved. Those who are unfamihar with 

 this important difference between mitotic and meiotic division, the 



