158 WALTER HEAPE. 



wliat bigger than the embryo drawn in Fig. 3, the medullary folds have not 

 advanced so far along the body of the embryo as they have in the latter 

 embryo. At the anterior end, however, they are slightly more advanced, and 

 where the folds meet in front a narrow slit is to be seen. The amnion is 

 shown covering the posterior half of the embryo, and the wide sinus rhom- 

 boidalis is indicated below it. 



Fig. 5, Stage F. — Transparent view of the same embryo, seen from below. 

 Four fully formed protovertebrse are present, and a fifth is indicated behind 

 the posterior one. The primitive streak projects into the sinus rhomboidalis. 

 The optic grooves appear as narrow lateral prolongations of the medullary 

 groove at its anterior end. The heart {ht.) commences to form at this stage, 

 and is indicated by a thickening of the blastoderm on either side the embryo 

 just behind and outside the optic grooves. 



Fig. 6, Stage G. — Surface view of embryo 2'33 mm. long. The meduUary 

 groove is closed up to the anterior end, where a small pore remains connecting 

 the medullary canal with the exterior. The sinus rhomboidalis is still widely 

 open behind. The head has now been folded off from the yolk-sac as far as 

 the line so.pl., which shows the point of divergence of the folds of somatopleure. 

 Faint indications of the divisions of the brain are shown, and the laterally 

 projecting optic vesicles are very distinct {op. v.). 



Fig. 7, Stage H. — Surface view of embryo 2*2 mm. long. Ten proto- 

 vertebra; are present. The closure of the medullary groove has advanced. 

 The sinus rhomboidalis is narrowed, and the primitive streak forced upwards 

 as a rounded knob at the posterior end of the latter. The head is more 

 rounded, and shows partial division into fore-, mid-, and hind-brains. The 

 amnion has been torn away, and the jagged edge of the somatopleure sur- 

 rounds the body of the embryo. 



Fig. 8, Stage H. — View of the under surface of the head of the same 

 embryo, showing the heart and the diverging folds of somatopleure and 

 splanchnopleure. 



Fig. 9, Stage J. — Surface view of an embryo 3"06 mm. long. The sinus 

 rhomboidalis is much narrowed, and the medullary groove closed for the 

 greater portion of its length. The optic vesicles and fore-, mid-, and hind-brains 

 are well shown. Thirteen protovertebrse are present. The primitive streak is 

 in the same condition as described for Fig. 7, also the amnion has been torn 

 away as it was in that figure. 



Fig. 10. — Lateral view of the head of the same embryo, showing the heart 

 and five visceral arches. 



Fig. 11, Stage E. — Median longitudinal section of the anterior end of an 

 embryo with three protovertebrse. The cephalic plate projects slightly over 

 the blastoderm in front, the folding-o£f process having already begun in this 

 embryo. The commencement of the fore-gut is indicated at al. c. A. small 

 portion of mesoblast exists between the epiblast and hypoblast of the blasto- 



