REPRODUCTION AND GROWTH i8i 



Accessory and unessential sexual characters. Certain characters 

 of significance in mating may distinguish the sexes. Horns, 

 antlers, differences of teeth, etc., are of this kind. Other sexual 

 characters are hair on the face, or elsewhere, in males but not in 

 females ; differences of size, colours, etc. There are innumerable 

 modifications of this kind and the significances of many accessory 

 and unessential sexual characters are obscure. 



Nutritive organs. Finally the female animals are usually 

 provided with organs for the carriage and nutrition of the embryos 

 and young. Such organs are brood-pouches, egg-sacs, marsupial 

 pouches, apparatus in connection with the egg-tubes, uteri, etc., 

 the placenta yolk glands, etc. Such accessory, nutritive organs 

 are very numerous and varied in forms. 



665. Fertilization. The germ-cells conjugate in an 

 analogous, or essentially similar way to the conjugation of two 

 unicellular organisms. 



For the moment we need not consider the processes by which 

 the germ-cells mature (see Section 8i). In all the gonidial cells 

 there are the same number of chromosomes. (This is not strictly 

 true, for there maybe reduplication of chromosomes, fragmentation 

 of chromosomes, supernumerary chromosomes, *' polyploidy," 

 linkages of chromosomes, or even actually inconstancy.) Sub- 

 sidiary hypotheses are devised to account for these deviations but, 

 for simplicity in exposition, we assume that the number of 

 chromosomes, in all the gonidial and somatic cells of all the 

 individuals of a species, is quite constant. 



In fertilization the gametes — say the ovum and spermatozoon 

 — come into contact. The sperm-head, which is practically the 

 " condensed " nucleus of the spermatozoon, penetrates the egg- 

 membrane and comes to lie in the cytoplasm of the ovum. 

 Hitherto the chromosomes have apparently been fused together 

 in the sperm-head, but now they are resolved into discrete bodies. 



At this stage, and as the result of maturation (Section 8i) 

 the number of chromosomes in the ovum-nucleus and the sperm- 

 nucleus is exactly one-half of the number in a gonidial, or somatic 

 cell. Say that there are 6, A B C D E F m the sperm-nucleus, 

 and a b c d e f in the ovum-nucleus. What happens next varies 

 considerably in different species, but we take what appears to be 

 the typical and essential series of events. 



The tgg' and sperm-nuclei (the pronuclei) lie side by side in 



