448 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



of gastrulation, viewed laterally (A) and from the basal pole (B), Fig- 

 ure 150, the map of the early anuran blastula, viewed laterally (A) and 

 dorsally (B). The general similarity is evident, the chief difference be- 

 tween the two being that in the anuran the regions of the axial organs do 

 not extend so far toward the apical pole as in the urodele, the greatest 

 difference being in the neural plate. In Figure 151 directions of cell mi- 

 grations (A and B) are indicated as they take place in gastrulation. With 



Fig. 150, A, B. — Regional map of early anuran blastula. A, lateral, B, dorsal, view; 

 denser broken lines, neural plate; less dense broken lines, general ectoderm; coarse stippling, 

 notochord; fine stippling, mesoderm; a, apical, v, basal, pole; /, beginning of invagination; b, later 

 blastopore; //, limit of invagination; g, gills; //, posterior limit of head ectoderm; mf, medullary 

 (neural) fold; e, field of eyes and chiasma; s, sucker; /, lens; an, auditory vesicle; pi, pronephros 

 and mesoderm of forelimb; somites indicated by lines in mesoderm (after Vogt, 1929). 



approach of gastrulation there is an increase in area and a stretching, 

 particularly in the apicobasal direction, in cells of the apical hemisphere, 

 apparently greatest in the dorsal region; and gastrulation begins with 

 the "rolling in," the invagination of cells in the median dorsal region at 

 the boundary between the future chorda-mesoderm and yolk. This in- 

 vagination extends laterally and ventrally until the blastopore becomes 



19260, b, 1929; Pasteels, 1932. Birds: Assheton, 1896; Peebles, 1898, 1904; Wetzel, 1925a, b, 

 1929, 1931, 1936; Kopsch, 1926a, b, 1934a, b; Graper, 1929; Pasteels, 19266, 19370; Jacobson, 

 1938 . For citation and discussion of the literature, including earlier papers concerning con- 

 crescence, gastrulation, and vertebrate developmental pattern in general, see the above 

 authors, particularly Vogt, 1929; Wetzel, 1931; Pasteels, 1937a; also Holmdahl, 1925, 1926, 

 1933, 1935; and textbooks of vertebrate embryology. 



