Qh STAGING OF AMPHIBIAN EMBRYOS 



60. - 61. Yolk exposed laterally on tail 'bud. embryo. 



62. Persistent yolk plug prevents tail formation in tail bud stage. 



65. - 66, Relation of persistent yolk to spina bifida. 



67. - 72. Spina bifida In various degrees. Note fair degree of head forma- 

 tion in some cases. 



