STAGE 19. Age 20 ¾ h.; length 1.5 -1.6 mm.

Ext. Crit.:*Initial neural tube stage*. *Neural folds touching each other*, except for inconstant openings at anterior and posterior end and behind nucal region. Considerable lateral extension of brain. Lateral outline of embryo still convex.

Int. Crit.: Embryonic pigment in epithelial layer of ectoderm dispersed over entire cell body. *Neural crest segregated into 4-5 cell masses; begining of lateral migration*.
*Segregation of anterior 4-6 somites*. Presumptive heart rudiment a single median mesodermal thickening.