FORM AND GROWTH 



119 



parts of the embryo came from various regions of the disk. The 

 results, as shown in Figure 64, confirm fully the His doctrine of 

 organ-forming germinal areas. The horizontal line through the 

 original center of the disk corresponds to the junction of the 

 head and neck. The early relative growth of the head, and the 



Figure 64. Location of the organ-forming germinal areas on the embryonic disk (blasto- 

 derm) of the chick, as shown by the experiments of Peebles. 40 The head region de- 

 velops from the front half of the disk (upper half in the figure), and the remainder of 

 the body from the rear (lower) half of the disk. 



later gradual differentiation of the trunk (up to the third day 

 of incubation) are also evident. Somewhat similar results were 

 obtained by Assheton 2 and others in embryos of various species. 



77. Comparative Embryonic Form 



A series of embryos, representing a comparable early stage 

 of development in the various classes of vertebrates, is shown 

 in Figure 65. As another representative of the primates (the 

 order to which man belongs), an embryo of a lemur (Tarsius) 

 may be seen in Figure 65A. The general features resemble 

 rather closely those of a human embryo of the same stage, the 

 most conspicuous difference being the longer tail of the lemur 

 embryo. Observe the same overgrowth of the head region, with 

 progressive decrease in the size of the body as we pass tailward. 

 The characteristic flexure of the body is also apparent. 



Another mammalian embryo is that of the pig (65B), which 

 presents the same general features as the human and lemur 



