Manchester Memoirs, Vol. I. (1906), No. 3. 7 



to rotate about a horizontal axis in a direction which is 

 the opposite of that in which the dorsal lip had moved 

 {Fig. I, E, F) ; and this rotation continues — the circle of 

 the blastopore becoming smaller all the time — until the 

 dorsal lip has returned, rather beyond the point from 

 which it started, to the (new) equator of the Qgg. The 

 angle subtended by the area of the yolk surface which it 

 traverses — both before and during the rotation — is about 

 75'^, as estimated by Kopsch, and the angle through 

 which the whole egg rotates about 100°. It follows that 

 the present vertical axis of the ^gg, which will be the 

 dorso-ventral axis of the embryo, makes the same angle 

 of 100° with the original egg-axis, that the animal pole is 

 situated below what will be the anterior end of the embryo, 

 and that the antero-ventral half of the embryo is developed 

 over the animal, the postero-dorsal half over the vegetative 

 hemisphere. {Pig- 2, ¥). 



It has been suggested from time to time that the 

 first movement of the blastopore, as well as the second, 

 is due to a rotation of the egg as a whole, the over- 

 growth of the pigmented over the unpigmented area being 

 only apparent, and the darkening of the vegetative hemi- 

 sphere due to the formation of pigment in the superficial 

 yolk-cells. Against this view it must be urged (i) that 

 Kopsch has seen the yolk-cells streaming underneath the 

 overgrowing lip ; (2) that if a small exovate is produced by 

 lightly puncturing the egg at the animal pole, this is after- 

 wards found in front of the medullary groove ; ^ (3) that 

 when the blastopore is experimentally prevented from 

 closing the anterior end of the embryo is seen to develope 

 at the animal pole ; and (4) that there is a cause, namely, 

 shifting of the yolk and consequent displacement of the 



^ Mr. Assheton has pointed oiU to me that this evidence is not 

 concUisive, as the exovates may become detached and shift their position. 



