PRIMITIVE 

 STREiX 



54. MORPHOGENETIC MOVEMENTS 



DETERMINED BY VITAL STAINING 



AND CHARCOAL PARTICLES 



Before beginning the following study of morphogenetic movements the student should re- 

 acquaint hin-iself with the various descriptions of the processes of normal development 

 from the earliest primitive streak stage (about 16 hours of incubation) to at least 42 hours 

 of incubation when the heart starts to beat. (See Lillie: "The Development of the Chick". ) 

 The pre-gastrulation stages are not available, since they occur within the oviduct 

 and every fertilized egg is at least in the primitive streak stage when layed. A condensed 

 survey is given below. 



The primitive streak, considered by many as homolo- 

 gous to the blastoporal lips of the amphibia, consists 

 of a longitudinal thickening of ectoderm extending 

 through from almost the anterior limit through about 

 2/3 of the length of the area pellucida. The primitive 

 groove is probably formed as a result of mesodermal 

 outgrowth, and beneath it all is a thin layer of endo- 

 derm lying on the yolk and attached to the streak only 

 at the level of Hensen's node. The bulk of the em- 

 bryonic tissues arise from material of this streak as 

 it becomes telescoped posteriorly in favor of an an- 

 teriorly elongating embryo. The margins of the area 

 pellucida and the area opaca together form the extra- 

 embryonic structures. 



At about 20 hours of incubation a head process ap- 

 pears anterior to the primitive streak, and this con- 

 sists of the anterior limit of the notochord and over- 

 lying medullary plate ectoderm. There is definite 

 cephalization (precocious development of the anterior 

 structures) but as the embryo lengthens the primitive 

 streak shortens (with the recession of its anterior 

 end, or Hensen's node). As the neural folds of the 



future brain region become approximated, the medullary plate is lengthened posteriorly 

 and the somites begin to appear, formed out of mesenchyme which was derived (by migra- 

 tion) from the sides of the primitive streak. The first pair of somites appear at about 21 

 hours of incubation and will be located at a position just posterior to the future otic ves- 

 icles. The first four somite pairs appear during the first day, all to be incorporated in 

 the head (occipital) musculature. The primitive streak is therefore not to be considered 

 as part of the embryo proper, but rather as a remnant of the blastoporal lips out of which 

 are derived the tissues of the embryo. 



Wetzel (1929) and Pasteels (1937) have mapped out the prospective organ-forming areas 

 of the primitive streak stage of the chick embryo, in a manner similar to that used by 

 Vogt (1926) and others on the amphibia, by the use of vital dyes. In fact, these investiga- 

 tors have found that there is surprising similarity in amphibian and avian morphogenetic 

 movements, and that the three major movements of convergence, invagination, and 

 elongation, are found in both. The notochord and the floor of the neural tube arise from 

 the material of Hensen's node, as shown by deep vital dye staining. The anterior end of 

 the primitive streak (exclusive of Hensen's node) gives rise to the lateral walls and roof 

 of the neural tube, and to the somites. The anterior part of the head arises from pre- 

 nodal materials. Vital staining and charcoal marking are useful procedures in determining 



Presumptive areas of the chick primitive 

 streak, modified from Willier & Rawles 

 193S: Proc. Soc. Exp. Biol. G Med. 

 32:1293. 



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