Manchester Memoirs , Vol. I. (1906), No. 3. 5 



its floor of about twenty layers of large polyhedral yolk- 

 cells. 



The first sign of the formation of the germ-layers is 

 given by the appearance of the structure known as the 

 dorsal lip of the blastopore {Fig. i, A). This is a short, 

 deeply pigmented groove, placed parallel to the equator, 

 and a little below it (about 25") at one point in the 

 boundary between the pigmented and the unpigmented 

 portions of the &^'g. With the appearance of the dorsal 

 lip the bilaterality of the ^^^ becomes fully established, 

 if it was not already in existence ; the plane which 

 includes the egg-axis and the dorsal lip is the sagittal 

 plane of the future embryo. 



The changes that now take place, as seen from the 

 vegetative pole, are as follows {Fig. i, B — D) : — The 

 groove begins to travel downwards over the surface of the 

 egg towards the vegetative pole, the area over which it 

 passes becoming covered by cells which are as deeply pig- 

 mented as those of the animal hemisphere. At the same 

 time the groove elongates, becoming crescentic ; in other 

 words, not merely one limited region of the boundary 

 between pigmented and yolk-cells, namely, the dorsal 

 lip, is involved in the process of overgrowth, but the 

 regions lying to the right and left of this, that is to say 

 the lateral lips of the blastopore, as well. As the dorsal 

 lip (the middle region of the groove) continues on its 

 course towards the vegetative pole, and as continually 

 fresh regions are drawn into the process at the sides, the 

 blastoporic lip becomes first semi-circular, and then three 

 parts of a circle, until, finally, when that region, the 

 ventral lip, which is diametrically opposite to the dorsal 

 lip also begins to grow down, it attains the form of a 

 circle enclosing the still uncovered portion of the vege- 

 tative hemisphere, the yolk-plug. The dorsal lip has now 

 moved down to, or a little beyond, the vegetative pole. 



