1904.] NATURAL SCIENCES OF PHILADELPHIA, 737 



stomodseum. At the anterior end is also seen the ectodermal 

 thickening which gives rise to the brain. 



Fig. 59. — Sagittal section of a much later stage than the last. The stomo- 

 dseum has assumed the position of the definitive mouth, while the 

 entoderm cells have multiplied and arranged themselves about the 

 rudiment of the intestinal lumen. Below the entoderm cells is seen 

 the mesoderm. 



rig.'60. — Sagittal section of a late stage. The entoderm cells now form 

 an epithelial layer about a well-defined lumen. The rudiment of the 

 intestine is making its appearance. 



Fig. 61. — Horizontal section of an embryo with four segments. 



Fig. 62. — Transverse section of a stage similar to that illustrated in Fig. 

 60, showing the relations of the three germ layers. 



