514 DEVELOPMENT OF PRIMITIVE BODY FORM 



mation in the tail and hindgut regions in the shark embryo, Squalus acanthias, 

 already mentioned, p. 484. In this species the entoderm of the developing tail 

 actually invaginates dorsad and closes in a manner similar to the forming 

 neural tube. That is to say, in the developing tail of the shark, two invagina- 

 tions toward the notochord are evident, one from the dorsal side, which in- 

 volves the formation of the neural tube, and the other from the ventral side, 

 effecting the developing enteric tube. 



The above facts suggest, therefore, that one of the main organizing influ- 

 ences at work during tubulation and primitive body formation emanates from 

 the pre-chordal plate area, the notochord, and the epimeric portion of the 

 mesoderm. From this general area or center, a chain of acting and interacting 

 influences extends outward, one part acting upon another, to effect the forma- 

 tive expression of the various parts of the developing body. 



J. Basic Similarity of Body-form Development in the Vertebrate Group 



of Chordate Animals 



In the earlier portion of this chapter, differences in the general procedures 

 concerned with tubulation and primitive body formation in round and flat- 

 tened gastrulae were emphasized. However, basically all vertebrate embryos 

 show the same tendency of the developing body to project itself upward and 

 forward in the head region, dorsally in the trunk area and dorso-posteriad in 

 the tail region. Literally, the embryonic body tends to lift itself up out of, 

 and above, the area which contains the yolk and extra-embryonic tissues. 

 This proneness to move upward and to protrude its developing head end 

 forward and its caudal end backward is shown beautifully in the development 

 of the embryos of the shark (figs. 229, 230), the mud puppy (fig. 227), the 

 chick (fig. 235C), and the pig (fig. 242). The embryo struggles to be free 

 from its bed of yolk and extra-embryonic tissue, as it were, and it reminds 

 one of the superb imagery employed by the poet, John Milton, in his im- 

 mortal poem, Paradise Lost, where he describes the development of the 

 lion thus: 



The grassy clods now calv'd; now half appear'd 

 The tawny lion, pawing to get free 

 His hinder parts, then springs as broke from bonds, 

 And rampant shakes his brinded mane. 



In summary, therefore, although it appears that rounded and flattened 

 gastrulae in the vertebrate group may have slightly different substrative con- 

 ditions from which to start, they all employ essentially similar processes in 

 effecting tubulation of the respective, major organ-forming areas and in the 

 development of primitive body form. 



