THE AXIS OF THE EMBRYO. 141 



turbing the ^ZZ- The ^^^ is hardy, and develops well in 

 shallow dishes holding sea-water about two inches deep. 



The membrane of the egg has a peculiar adhesive disk, about 

 3 mm. in diameter, which has a constant position, with the 

 centre of the disk at the vegetative pole, directly opposite the 

 micropyle. By means of this disk the ^g^ is firmly glued to 

 the supporting surface, usually the underside of a rock, the 

 inside of a broken jug, tin can, a piece of stovepipe, or even 

 an old boot-leg. The disk consists of a transparent secretion, 

 which becomes opaque and gluey on contact with water. It is 

 of nearly uniform thickness, and is closely applied to the egg- 

 membrane everywhere except for a narrow margin which pro- 



FiG. I. Fig. II. 



jects all around as a thin rim (Fig. I). The disk is saucer- 

 shaped, and only a little thicker thaTi the egg-membrane itself. 

 I have been able to separate it from the membrane in the case 

 of eggs hardened before attachment. As the Qgg is generally 

 fastened to more or less plane surfaces, it appears strongly 

 flattened on the side of attachment, as described by Dr. Ryder 

 and as shown in Fig. II. 



That the egg-membrane, with its adhesive disk, does not 

 change position after attachment is, then, very certain ; but is 

 the egg itself also fixed within the membrane, and does it main- 

 tain a fixed position for the time required to reach an early 

 stage in the axial differentiation — six or seven days after fer- 

 tilization } or is it liable to rotate and get displaced, and so 

 invalidate the results obtained .-" 



In the experiment of '91 the eggs were allowed to flow from 



