HUBRECHT. IPL NE 
Fig. 66 to 70. Five sections through the 
very earliest stages of Ornithorhynchus and 
Echidna. Fig. 66 earliest cleavage stage. Fig. 67 
visible separation of trophoblast cells 7 and of mothercells of the embryonic 
knob, e& Fig. 68 and 69 this separation is yet far more clearly established: 
the trophoblast 7 having travelled much further over the yolk surface, the em- 
bryonic knob (ck) being partly imbedded in the yolk; Fig. 70 a yet further 
stage in which ecto- and entoderm (ec and ev) have become differentiated by de- 
Jamination and in which the ectoderm has come to the surface; ¢ trophoblast. 
(Fig. 66, 68, 70 after Wilson and Hill, ‘07; Fig 67, 69 after Semon, 94.) 
