Page:Popular Science Monthly Volume 85.djvu/241

Rh the egg. If an egg is fertilized by a sperm which lacks the X-chromosome 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 distribution of the chromosomes and the occurrence of certain characters of the adult animal. The association of maternal and paternal chromosomes 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 characters (Boveri); sex determination is associated with the distribution of a particular chromosome to one half of the spermatozoa, and the fertilization 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).

On the other hand, the most direct and the earliest recognized correlations 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.

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 becomiesbecomes [sic] 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).

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