PLATE IV 



ENTIRE EGGS OF HETERODOHTUS JAPOHICUS 



SHOWING THE RELATIONS OF THE EMBRYO AND 



THE OVERGROWTH OF THE YOLK MASS BY THE BLASTODERM 



Figs. 40 to 42. Upper hemisphere of eggs taken shortly after deposition. The germinal disc or "orange spot" 

 is presumably undergoing cleavage. In Figures 40 and 41, the disc is surrounded by concentric 

 white zones; in Figure 42, by a single white zpne (periblast?). 



Fig. 43. A slightly older egg in which the germinal disc is in a late blastula stage. 



Fig. 44. The germinal disc or blastoderm is now almost ready for gastrulation. 



For larger and more accurate drawings of blastoderms in approximately the same stage, see Figure 11, plate I, and 

 Figure 80, plate VII. 



Fig. 45. An egg sUghtly later than the preceding. The blastoderm is larger, and is circular in outline. 



Fig. 46. This blastoderm is much larger than the preceding one. Its posterior (lower) margin is upraised 

 and is in a very early stage of gastrulation. See also Figure 1 2, plate I. 



Fig. 47- A still larger blastoderm showing at its posterior edge the upraised embryonic area. For details 

 see Figures 13 and 14, plate I. 



Fig. 48. There is shown here a marked increase in the size of the blastoderm. The embryo shows a definite 

 head region. An embryo in approximately the same stage is shown in detail in Figure 16, plate I. 



Figs. 49 to 51. These figures show the blastoderm spreading over a hemisphere of the egg while the embryo, 

 situated at the slight notch in the posterior margin of the blastoderm, is still very small and cannot 

 be accurately dehneated on this scale. The three stages are probably equivalent to Figures 22, 

 24, and 25, plate II. 



The drawings of this plate are in natural colors, save that the embryos represented in Figures 49 to 51, which in life are 

 colorless and translucent, are portrayed in opaque white. All the drawings are reproduced about natural sise. 



In all the figures, the yolk mass shows the furrows interpreted, by Dean, as a reminiscence of holoblastic cleavage. 



