THE CELLULAR BASIS OF HEREDITY 237 



the egg. If an egg is fertilized by a sperm which lacks the X-ebromo- 

 some a male is produced; if fertilized by the other type a female results. 

 (7) Finally, Morgan has found that there is a linkage of certain somatic 

 characters with sex in the fruit fly, Drosophila, which can be readily 

 explained by assuming that the determiners for these characters are 

 in some way associated with the sex chromosome. 



We have in these facts a remarkable correlation between the dis- 

 tribution of the chromosomes and the occurrence of certain characters 

 of the adult animal. The association of maternal and paternal chromo- 

 somes in fertilization and their segregation in the maturation of the 

 germ cells is parallel to the association of Mendelian characters in the 

 zygote and their segregation in the gametes; if the distribution of 

 chromosomes in cleavage is abnormal the larva shows abnormal char- 

 acters (Boveri) ; sex determination is associated with the distribution of 

 a particular chromosome to one half of the spermatozoa, and the fertil- 

 ization of the egg by one type or the other of spermatozoa (Wilson) ; 

 the linkage of certain characters with sex finds a ready explanation by 

 assuming that these characters are associated with the sex chromosome 

 (Morgan). 



2. Cytoplasmic Correspondences 



On the other hand, the most direct and the earliest recognized cor- 

 relations between the oosperm and the developed animal are found in 

 the polarity and symmetry of the fertilized egg and of the animal to 

 which it gives rise. 



(a) Polarity 



In all eggs there is a polar differentiation, one pole, at which the 

 maturation divisions take place, being known as the animal pole, and the 

 opposite one being known as the vegetative pole. The substance of the 

 egg in the vicinity of the animal pole usually gives rise to the ectoderm, 

 or outer cell layer of the embryo; the portion of the egg surrounding 

 the vegetative pole usually becomies the endoderm or inner cell layer. 

 The axis which connects these poles, the chief axis of the egg, becomes 

 the gastrular axis of the embryo and in every great group of animals 

 bears a constant relationship to the chief axis of the adult animal. The 

 polarity of the developed animal is thus directly connected with the 

 polarity of the egg from which it came (Figs. 23, 26, 29, 30, 40, 41). 



(b) Symmetry 



In many cases the symmetry of the developed animal is foreshadowed 

 in the symmetry of the egg. The eggs of cephalopods (Fig. 40) and of 

 insects (Fig. 41) are bilaterally symmetrical, while they are still in the 

 ovary; in other cases, such as ascidians, amphioxus and the frog, 

 bilateral symmetry appears immediately after fertilization (Fig. 29, 1, 

 2), though in some of these cases there is reason to believe that the 



