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DESCRIPTION OF PLATE XIII. 



Fig. I. Human embryo of about the fourth week, natural size. 

 Fig. IT. Do., magnified five diameters. 1, head ; 2, spine ; 3, anterior extremity ; 

 4, posterior do. ; 5, coccyx ; 6, rudimentary eye ; 7, rudimentary ear ; 

 8, external meatus ; 10, hepatic mass. 

 Note. — This fig. is shown as under the combined effects of transmitted and 

 reflected light. 

 Fig. III. Figs, up to 10, as in the former; 11, rudimentary pulmonic vesicles; 12, 

 auricle; 13, ventricle; 14, bulbus arteriosus 

 Note. — This fig. is shown only as an opaque object. The internal ear and 

 eye and the vertebral markings are consequently not seen. 

 Fig. IV. Heart detached ; a, auricle ; b, ventricle ; c, bulbus arteriosus. 

 Fig. V. Heart of embryo at beginning of second month. 

 Fig. VI. Do., end of second month. 



TRANS. MIC. SOC. VOL. III. 



