No. 2. J FORMATION OF THE FISH EMBRYO. 42 I 



complicated processes that are most difficult to interpret. The 

 first and most important change involves the separation of the 

 three main portions of the blastoderm from the unspecialized 

 cap of cells. The embryonic portion, the germ-ring, and the 

 extra-embryonic area appear. 



The cap, as it flattens, shows very early a thicker peripheral 

 ring that gives a strong contrast v^rith the relatively thinner 

 central portion. This ring, almost from the beginning, per- 

 haps even at the beginning, is slightly broader over a portion 

 of its extent, and this gives in the living embryo the first in- 

 dication of the embryonic side of the disc (PI. XXIII, Fig. i). 

 At this region of thickening a tongue of cells pushes up 

 rapidly beneath the surface towards the apex of the disc. 

 Coincident with this event that portion of the cap lying 

 above the tongue becomes thicker, as shown in outline in 

 PI. XXIII, Fig. 2. This thickening, as my experiments show 

 very conclusively, reaches quite up to the apex of the early 

 blastoderm. 



A clearer region of the blastoderm, as seen in surface views, 

 is now left between the embryonic portion and the germ-ring. 

 This extra-embryonic region is broadly semi-lunar in outline, 

 as the figures show (PI. XXIII, Fig. 2). 



The embryonic portion is continuous at its sides with the 

 germ-ring, which has also become more sharply defined during 

 this time. 



Serial sections through the blastoderm show to some extent 

 how these changes are brought about. At first the blastoderm 

 is of equal thickness throughout. As it begins to flatten, its 

 center becomes relatively thinner than the periphery. The 

 thicker edge marks the beginning of the germ-ring. In addi- 

 tion to its greater thickness the germ-ring shows a tendency 

 to push certain of its cells under the others, forming in section 

 a small tongue of cells that are continuous with the more de- 

 veloped tongue of the embryonic region. 



This process of inturning — whether brought about by an 

 active invagination or as the result of a process of overrolling 

 of the edge I cannot determine definitely — takes place more 

 rapidly and on a somewhat larger scale in the embryonic 



