"Longitudinal vertical section of frog embryo, shortly before closure of blastopore. FB., fore-brain; EC., ectoderm; N., notochord; SC., canal of spinal cord; NE., nerenteric canal; B., blastopore; M., mesoderm; P., beginning of pituitary invagination." -Thomson, 1916

Frog Embryo

"Longitudinal vertical section of frog embryo, shortly before closure of blastopore. FB., fore-brain;…