Plate IV. 



Figs. 37 and 38. Later stages of closure of medullary folds. 



Fig. 39. Caudal eud of stage shown in Fig. 38. 



Fig. 40. Beginning of head, formation of myotomes. 



Fig. 41. Later stage than Fig. 40. First indication of gill arches. 



Fig. 42. Same stage as in Fig. 41. Caudal end of the embryo. 



Fig. 43. Later stage, eyes well formed, also gill arches. 



Fig. 44. Head of a later stage. 



Fig. 45. Same stage as Fig. 44, turned so that both head and tail ends are 

 seen. 



Fig. 46. Same stage as Fig. 44, back view showing myotomes. 



Figs. 47 and 48. Later embryonic stage, two views. In this stage, the eyes, 

 nasal pit, gill clefts, heart and limb buds, may be seen. 



