Fig. 217. Primary tubes (tubulations) of the primitive vertebrate body. (A) Sche- 

 matic representation of epidermal, neural, and entodermal tubes in the early embryo of 

 the shark. Observe that a well-developed, post-anal or tail gut continues to the end of 

 the tail. (B) Gut, neural, and epidermal tubes in the amphibian type. (C) Gut, 

 neural, and epidermal tubes in the chick and mammal type. (D) Mesodermal tube in 

 the shark embryo. (E) Mesodermal tube in the amphibian embryo. (F) Mesodermal 

 condition in the early bird and mammal embryo. (G) Transverse section of shark 

 embryo, showing tubulations of major organ-forming areas and primary coelomic con- 

 ditions. (H) Transverse section of frog embryo shortly after closure of neural tube, 

 showing the five fundamental body tubes oriented around the notochord. 



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