420 MORGAN. [Vol. X. 



ditions the embryo might not have completed itself by methods 

 other than those followed in the normal development. Indeed, 

 in the same communication I showed that, when one of the 

 first two cells was removed, the embryo still formed, although 

 of smaller size. 



The present paper is an attempt to meet this possible criti- 

 cism. In addition, by a detailed examination of the normal 

 phenomena, I have tried to show that my conclusion drawn 

 from the experiment was a legitimate inference. 



The processes by which the protoplasmic segmented cap, or 

 blastoderm, of the fish-egg surrounds ultimately the sphere of 

 yolk on which it rests may be first briefly reviewed. 



The cap lying at one pole of the yolk-sphere is a convexo- 

 concave disc, circular in outline, with its concave side turned 

 towards the yolk. The cap begins to flatten, and, in conse- 

 quence, covers a larger surface of the yolk-sphere. The process 

 of flattening and overgrowth continues until the equator is 

 reached, during which time the perimeter of the cap is enlarg- 

 ing. After the equator is passed, the perimeter of the over- 

 growing blastoderm must steadily diminish in length, while 

 the area covered continually grows larger. When the yolk is 

 covered the perimeter of the disc is reduced to a point and 

 obliterated. The yolk has been overgrown, and the area of the 

 blastoderm has become that of the yolk-sphere enclosed. 



Two other phenomena play a conspicuous part during this 

 process of overgrowth. Along one of the meridians of the 

 sphere the embryo forms, with its head resting at the pole 

 around which the blastoderm started. The embryo increases 

 in length along its meridian pari passu with the overgrowth of 

 the blastoderm, and its posterior end is always continuous with 

 one portion of the perimeter of the blastoderm. 



The perimeter of the cap, or blastoderm, is thicker than the 

 rest of the cap (except in the embryonic portion), and this 

 thickened zone is spoken of as the germ-ring. The germ-ring 

 is a conspicuous feature of the development, and is only obliter- 

 ated when the overgrowth is completed. 



If we examine in detail the method by which the changes 

 sketched above take place, we find them due to a series of 



