274 LILIAN SHELDON 



Figs. 5, 6, and 7.— Tliree transverse sections through an ovum in which 

 the blastopore is well formed. 



rig. 5. Through the blastopore. 

 Fig. 6. Just behind the blastopore. 



Fig. 7. Near the posterior end of the primitive streak. 



Fig. 8. — Transverse section tJirough an ovum with a primitive streak. 



Fig. 9. — Transverse section through a portion of the same ovum, to show 

 the invagination at the blastopore beginning at the anterior end of the primi- 

 tive streak. Drawn under a higher power than Fig. 8. 



Fig. 10. — Transverse section through an embryo with appendages, to show 

 the duct of the first somite opening to the exterior. 



Fig. 11. — Transverse section through the same embryo four sections pos- 

 terior to Fig. 10, to show the duct opening into the first somite. 



