METABOLIC GRADIENTS 69 



geal region, which in the adult is by far the larger part of 

 the body, is very short and consists of but few cells. 

 This major gradient is very distinct, but the ventro- 

 dorsal gradient is also evident. The section shows 

 that multiplication of cells and structural development 

 are proceeding chiefly in the ventral region, while the 

 dorsal region consists of relatively few cells. Examina- 

 tion of transverse sections of embryos would show the 

 transverse gradients: we should find that the develop- 

 ment was proceeding more rapidly in the median ventral 

 region than in the lateral regions. The transverse 

 and the ventro-dorsal gradients are in reality different 

 components of the same gradient. The fact is that a 

 developmental gradient extends laterally and dorsally 

 from the median ventral region. In such a bilaterally 

 symmetrical animal there are then two chief develop- 

 mental gradients, a major, from the anterior region 

 posteriorly, and a minor, from the median ventral, or in 

 some cases most of the ventral region, laterally and 

 dorsally. In other bilaterally symmetrical inverte- 

 brates relations are in general similar. In Fig. 2 (p. 38) 

 the relations in a simple case of this sort are diagram- 

 matically indicated. 



In the vertebrates the longitudinal gradient is 

 similar to that in the invertebrates, but instead of a 

 ventro-latero-dorsal gradient, as in the invertebrates, 

 the gradient is dorso-latero- ventral in direction. Fig. 14 

 represents an early stage of the chick embryo in which 

 the head is just becoming morphologically distinct, but 

 other organs are not yet formed, while in Fig. 15, a 

 later embryonic stage, the head region is advanced 

 in development, and differentiation of the body is 



