GASTRULATION IN VARIOUS CHORDATA 445 



In figure 213A, B, and C, two general areas of entoderm are shown: 



(a) that exposed at the surface (cf. fig. 179), and 



(b) the entoderm lying in the deeper areas of the blastoderm (cf. fig. 179, 

 cells in black). 



According to Vandebroek, '36, the deeper lying entoderm is extra-embryonic 

 entoderm (in fig. 213, this deeper entoderm is represented as a black area with 

 fine white stipple), whereas the entoderm exposed at the caudal portion of 

 the blastoderm in figure 179 A and B, and figure 213A is embryonic entoderm. 



The later distribution of the major presumptive organ-forming areas of 

 the shark blastoderm is shown in figure 213E-M. In figure 213, observe the 

 periblast tissue connecting the blastoderm with the yolk substrate. 



As the notochordal, entodermal, and mesodermal cells move inward during 

 emboly, the presumptive epidermal and neural areas become greatly expanded 

 externally by the forces of epiboly as shown in figures 213B-E, and 213H. 

 (Compare the positions of these two areas in fig. 179B.) 



The general result of the gastrulative processes in the shark group is to 

 produce a blastoderm with three germ layers similar to that shown in figure 

 21 3L and M. The notochordal and pre-chordal plate cells occupy the median 

 area below the neural plate as shown in figure 213E and F; the mesoderm 

 and entoderm lie on either side of the median notochord as shown in figure 

 213M. A little later the entoderm from either side of the notochord grows 

 mediad to establish a complete floor of entoderm below the notochord as repre- 

 sented in figure 213L. 



7. Intermediate Types of Gastrulative Behavior 



In certain forms, such as the ganoid fish, Amia, and in the Gymnophiona 

 among the Amphibia, the gastrulative processes present distinct peculiarities. 

 In general, gastrulation in the bony ganoid fish, Amia calva, presents a con- 

 dition of gastrulation which is intermediate between that which occurs in the 

 teleost fishes and the gastrulative procedures in the frog or the newt. For 

 example, a blastodisc-like cap of cells is found at the end of cleavage in the 

 bony ganoid. This cap gradually creeps downward around the yolk masses 

 which were superficially furrowed during the early cleavages. This process re- 

 sembles the cellular movement occurring during epiboly in teleost fishes. In 

 addition, the entodermal, notochordal, and mesodermal materials migrate in- 

 ward in much the same way as occurs in the teleost fishes, although the forma- 

 tion of the primitive archenteron resembles to a degree the early invaginative 

 procedure in the frog. However, a distinctive process of entodermal formation 

 occurs in Amia, for some of the entodermal cells arise as a separation from 

 the upper portion of the yolk substance where yolk nuclei are found. (See 

 fig. 214D, E; consult Eycleshymer and Wilson, '06.) 



The gastrulative processes in the gymnophionan Amphibia are most pe- 



