The Embryology of Heterodontus japonicus 



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Text-figure 51. 

 Sagittal section of a blastoderm o{ Heterodontus phillipi in a stage in which the segmentation 



cavity is well established. 



ant, anterior; para, parablast or periblast. 

 After Haswell, 1898, Fig. 4, pi. IV. 



series of sagittal sections of a gastrula stage. It has therefore been necessary to interpret 

 Dean's drawings in the light of what is known concerning the development of other 

 species of Elasmobranchs. The most helpful contributions are those of the Zieglers (1892 

 and 1902); Haswell (1898); Riickert (1885 and 1899); and Scammon (1911). 



Among Dean's drawings, the one reproduced as Figure 11, plate I, represents the 

 earliest blastoderm that shows indications of gastrulation. This blastoderm is decidedly 

 elongate — a transient phase in its development. Its margins, constituting the embryonic 

 rim or germinal ring, are sHghtly upraised, particularly at the posterior (lower) end. At 

 this end a small and rather indistinct pit indicates the site of beginning invagination; but 

 the pit may be an artifact. The central portion of the blastoderm retains some of the 

 reddish color characteristic of the germinal disc of an earlier stage. The pale-yellowish 

 zone surrounding the blastoderm is broadest at its anterior (upper) end. It represents the 

 marginal portion of the periblast. The original figure bears the notation ''5 mm." in 

 Dean's handwriting. This probably refers to the length of the blastoderm. 



Figures 45 and 46, plate IV, show the rapid disappearance of color within the 

 blastoderm except for a narrow line along its border. They show also the change from an 

 elliptical outline to one that is approximately circular. Figure 12, plate I, represents a 

 blastoderm a little older than that shown in Figure 11, plate I. It is only moderately ellip- 

 tical, and the presence of an upraised portion at the posterior end constituting the so- 

 called embryonic shield indicates that it is a gastrula. The median groove traversing the 

 ectoderm of the embryonic shield is the neural groove. Figure 13, plate I, is slightly 

 later and corresponds to Figure 47, plate IV, which shows the entire egg in color. Here, 

 and in the following stage (Figure 48, plate IV), one notes the extension of the blast' 

 oderm over the surface of the yolk. That this extension is relatively rapid may be de- 

 duced from the slight increase in the size of the embryo proper between the stages repre- 

 sented by Figures 46 to 48, plate IV. 



Figure 14, plate I, represents the first of a series of embryos of H. japonicus detached 

 from the yolk mass, stained, cleared and mounted in toto. These embryos may have been 



