General Features 15 



very young fish is perfectly symmetric but, as growth proceeds, the 

 entire head-region undergoes a twisting or rotation, the result of which 

 is that, in the adult fish, the eye, gills, and other organs of that side on 

 which the fish lies are shifted around to the exposed upper surface of 

 the fish (Fig. 9). Thus, if the fish is one which lies on the left side, the 

 left eye appears in the same flat upper surface with the right side of the 



Fig. 7. Ventral views of the heart of chick 

 embryos at successive stages to show its changes 

 of shape and its regional diflerentiation. (Cour- 

 tesy, Patten: "Early Embryology of the 

 Chick," Philadelphia, The Blakiston Company.) 



body. This distortion of the head is clearly an adaptation to the pe- 

 culiar habit of the fish and obviates impairment of the functions of im- 

 portant organs which otherwise would be pressed down against the 

 sea bottom. 



Comparing the several systems of organs, it is evident that the pri- 

 mary symmetry of the early embryo is retained in the motor mecha- 

 nism of the body (that is, the body-mnscles), the corresponding parts 

 of the nervous system, and the skeleton. It is of advantage to the ani- 

 mal that its capacity for bodily activity should be equally developed 

 on the two sides of the body. But the primary symmetry may be modi- 

 fied or lost by organs in which symmetry has no functional importance. 

 So long as an organ receives an adequate blood-supply, it does not 

 matter whether the blood-vessels serving that organ are symmetrically 

 arranged or not. So long as the animal obtains enough oxygen, it does 

 not matter as to the shape, size, and arrangement of the several lobes 

 of the lungs. Glandular organs such as the liver and pancreas may be 

 extremely irregular in form. The liver, as it develops, grows back 

 against the digestive tube and occupies such space as is left available 

 to it by the unsymmetrieally placed stomach and intestine. 



Bilateral symmetry, therefore, is a feature of the "plan" of a \ erte- 

 brate as laid out in the lines of the early embryo. But in the course of 



