190 Heredity and Environment 



mined most accurately (Fig. 67). Thus not only does he locate 

 particular genes in particular chromosomes but he is able to locate 

 the relative positions of these genes in each chromosome. This 

 is in all respects the most remarkable work which has ever been 

 done in this field ; for the first time it gives us a detailed picture 

 of what Weismann called the "architecture of the germ plasm," 

 — for the first time it assigns to different genes "a local habita- 

 tion and a name." 



3. Cytoplasmic Inheritance. — The most direct and the earli- 

 est recognized correlations between the oosperm and the devel- 

 oped animal are found in the polarity and symmetry of the egg 

 cytoplasm and of the animal to which it gives rise. 



(a) Polarity. — In all eggs there is 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 becomes 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 it bears a constant relationship to the chief axis of the 

 adult animal. The polarity of the developed animal is thus di- 

 rectly connected with the polarity of the egg from which it came 

 (Figs. 42, 45, 46, 47, 68,69). 



(b) Symmetry. — In many cases the symmetry of the developed 

 animal is foreshadowed in the cytoplasm of the egg. The eggs 

 of cephalopods (Fig. 68) and of insects (Fig. 69) are bilaterally 

 symmetrical while they are still in the ovary ; in other cases, such 

 as ascidians, Amphioxus and the frog, bilateral symmetry ap- 

 pears immediately after fertilization (Figs. 9, 10, 46, 47), though 

 in some of these cases there is reason to believe that the eggs are 

 bilateral even before fertilization ; in still other cases bilaterality 

 does not become visible until later in development and we do not 

 now know whether it is present in earlier stages or not ; but wher- 



