544 VERTEBRATE LIFE AND ORGANIZATION 



Somite" 



Nephrogenic 



"Amnion- 

 -Neurad fold- 



Late rail y^ 



Embryonic 

 Coelom. 



Extra. - 

 ^embryonic 

 co<z.Jom 



Somite 



Nephrogenic 

 ridOe 



Lateral plate 



Dorsal aorta." 



Dorsal 

 mesentery /5f 



Myotome 

 Genita.1 ridoe 



Somite: — 

 ^Developin.^ dermis 

 ^uT Developing 



vertebra. 

 I Myotome^ 



Kidney 

 (Ne-phrodenic 

 ridrfe) 



Peritoneum 

 Mesentery 



Lateral plate : -^ 



^rr-Dorsa.1 pa-ncreas 



Livfey 



Figure 31.6. Diagrammatic cross sections through vertebrate embryos of different 

 ages. The separation of the embryo from the yolk sac, the differentiation of the meso- 

 derm, and the formation of the Hver and dorsal pancreas are shown. (Modified after 

 Patten.) 



bryonic skeletal muscle blocks, or myotomes. The myotomes extend out 

 between the surface ectoderm and the lateral plate and develop into 

 most of the musculature of the body wall and appendages (somatic 

 muscles). The segmentation of the muscles is retained in adult fishes 

 but muscle segmentation is largely lost during the later development of 

 most higher vertebrates. 



The resemblance of certain of the embryonic stages of the higher 

 vertebrates to the adults of lower vertebrates, such as we see in the 

 segmentation of the muscles, is regarded as strong evidence for evolution. 

 In the late nineteenth century, Ernst Haeckel postulated that embryos 

 pass through stages during their embryonic development (ontogeny) that 

 their ancestors passed through during evolution (phylogeny). In other 

 words, "ontogeny recapitulates phylogeny, or the embryo climbs its own 

 family tree." This generalization is no longer taken as literally as 

 Haeckel intended. It is now clear that the embryos of higher vertebrates 



