Manchester Memoirs, Vol. /. (igo6), No. li. g 



centre of gravity, to account for the rotation in the 

 second case, but no such obvious explanation of the 

 supposed rotation in the first. 



The whole of the process which has been described is 

 no more nor less than the closure of the blastopore ; the 

 blastopore, which is indeed but a virtual blastopore at the 

 start, being that circular subequatorial line along which 

 the pigmented animal pass into the unpigmented vege- 

 tative cells. And clearly this closure is bilateral, taking 

 place as it does most rapidly at the dorsal lip, least rapidly 

 at the ventral lip, and at an intermediate rate at the lateral 

 lips in between. The examination of sections will now 

 show us that the closure involves (i) a movement of 

 the vegetative cells into the segmentation cavity together 

 with (2) an overgrowth and ingrowth of cells at the blasto- 

 poric lip resulting in the formation of a new cavity, for 

 which we will retain the time-honoured expression of 

 ' archenteron ' ; and that during the process the material 

 for the germinal layers is brought into position and laid 

 down. 



A sagittal section of the egg passing through the dorsal 

 lip at its first appearance {Fig: 2, A) shows the groove 

 placed about 25^ below the equator in the zone of inter- 

 mediate cells. The cells which immediately bound the 



B. The dorsal lip further advanced. The yolk-cells beginning to 

 creep up into the segmentation cavity. 



C. A slit-like archenteron formed {arch). 



D. The dorsal lip has reached the vegetative pole ; the ventral lip 

 (&./.) is just formed. Mes. 2. mesoderm formed from yolk-cells pushed 

 into segmentation cavity ; y.p. yolk plug. 



E. The egg is beginning to rotate. The segmentation cavity is 

 reduced, the archenteron enlarged. 



F. Rotation completed. Mes. v. mesoderm at ventral lip ; s.c. 

 remains of segmentation cavity. 



In all the figures the arrow marks the egg axis, the head of the arrow 

 the animal pole. 

 (Original. ) 



