Manchester Memoirs, Vol. I. ( 1 906), No. 3. 1 1 



over the dorsal lip the yolk-cells at the equator may be 

 observed creeping up into the segmentation cavity, along 

 the under surface of its roof As this movement begins 

 so it takes place most rapidly on the dorsal side, but it is 

 continued none the less laterally and ultimately ventrally 

 as well. The segmentation cavity is thus reduced to a 

 small space situated ex-axially, and on the ventral side 

 i^Fig. 2, E). The archenteron meanwhile is extended in 

 the direction of the animal pole {Fig. 2, D). This exten- 

 sion may be due to a ' splitting ' amongst the yolk-cells, 

 or (more probably) to an actual invaginatory process ; 



Fig. 3. 

 Transverse sections of the Frog's egg. 



A. Before rotation during overgrowth of the lips of the blastopore. 

 /./. lateral lips. The yolk-cells are creeping up into the segmentation cavity. 



B. After rotation. The archenteron is widened. n.cJi. notochoid. 

 (Original.) 



but in any case a greater change soon follows in its dimen- 

 sions. The yolk-cells which form its floor are now pushed 

 up towards the animal pole, and inwards towards the egg 

 axis, so that the cavity gains simultaneously in length, 

 depth, and breadth {Fig. 2, E, F ; and Fig. 3, B) ; it now 

 extends into the animal hemisphere, being separated by 

 only a thin partition of yolk-cells from the segmentation 

 cavity. The fate of the latter appears, as Professor Keibel 



