No. 2.] FORMATION OF THE FISH EMBRYO. 427 



the germ-ring, during its overgrowth, goes through a series of 

 changes. When it is first established, its breadth is shown in 

 Fig. I, A, and Fig. 2, A. When it has extended some dis- 

 tance over the yolk, as shown in Fig. 3, B, we find that it has 

 diminished a little in breadth. When the germ-ring has just 

 covered the equator of the &g^, as shown in Fig. 4, A, we find 

 that its breadth has very much diminished. An examination 

 of stages intermediate between the last two, shows the process 

 to be due to a continuous and gradual change. We see then 

 clearly that while the germ-ring is increasing in length it 

 decreases in breadth — just as an elastic band would change if 

 drawn over a sphere. Moreover, if we artificially remove 

 a part of the yolk during these stages, we find that the germ- 

 ring is highly elastic, and contracts at once to fit the smaller 

 sphere. 



After the equator is passed we find that the germ-ring 

 slowly increases in superficial breadth again, as Fig. 5, B, and 

 Fig. 6, C, show. Again we find a very exact resemblance to 

 an elastic band, under similar circumstances. 



Sections cut exactly at right angles to the length of the band 

 are most instructive. The section drawn in PI. XXIV, Fig. 

 13, B, corresponds to one side of the whole section shown in 

 Fig. 13, A. The edge of the blastoderm is somewhat thick- 

 ened, and the cells show a tendency to turn under. 



The next figures are taken from an embryo at 11 a.m. 

 (Fig, 2, A). Figs. 14, A and B, show respectively longitudinal 

 and cross sections through the blastoderm, and C and D, more 

 magnified portions of the germ-ring. At this stage the center 

 of the blastoderm has become thinned out, leaving the edges 

 of the blastoderm, that form the germ-ring, in the condition 

 shown in the figures. We find that the germ-ring, both in 

 front {C) and at the sides (Z>), is composed of a large number 

 of cells, and has a very considerable breadth. We find, more- 

 over, that the cells of the upper layer (beneath the ' covering- 

 layer ') gradually pass into the cells of the extra-embryonic 

 region. The most important point that such figures show, is 

 that the anterior portion of the germ-ring has less breadth, and 

 is composed of a fewer number of cells than the sides of the 



