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line between yolk and ectoderm. These intercellular spots vary in 

 size and shape — are frequently angular. In Fig. 3, precisely five 

 spots had appeared, the middle one the largest, and all between 

 yolk cell {y. c.) and the ectoderm cells facing it. The lining up of 

 the ectoderm cells, and appearance of intercellular pigment spots, 

 continues on each side of the locality where the lip first appears; 

 and one half hour later than the time at which Fig. 3 was drawn, 

 the same embryo presented the appearance shown in Fig. 4. The 

 •'lining up" and the development of pigment spots are two separate 

 processes. This is shown by the occasional lining up of cells, some 

 little time before the appearance of the spots, as is the case in Fig. 3'. 

 The lining up itself must be due to some slight changes in the 

 shape of ectoderm cells — changes too subtile to follow. The pigment 

 spots which become more and more numerous and crowded, even- 

 tually fuse to form a continuous line, which is at first rough and 

 jagged, but which gradually becomes smooth — compare Figs. 3, 4, 

 5 (a, b, c) successive stages of the same egg. The spots, on careful 



focussing, present an 

 appearance such as 

 suggests them to be 

 gaps or depressions, 

 caused by the actual 

 separation of cells, the 

 opposed surfaces of 

 which are pigmented, 

 or develop pigment as 

 they pull apart. The 

 study of sections of the 

 normal egg bears out 

 this interpretation. As 

 these intercellular gaps 

 increase in number 

 along the line of de- 

 marcation, they unite and form the well known furrow, which bounds 

 the blastopore hp. The intercellular pigment spots (gaps) are not 

 strictly confined to the line of the blastopore lip. A relatively 

 small number make their appearance, in the neighborhood of the 

 blastopore lip, lying between the cells of the ectoderm, and also 

 between the yolk cells (Figs. 3 and 4). 



The lateral extension of the blastopore lip, with final formation 

 of its mid-ventral portion, takes place in the same way as the 



a 



t 



Fig. 5. Three later aud successive stages (a, b, c) 

 in the development of the egg shown in Figs. 3 and 4. 

 Halt' of the upper surface of the compressed egg is shown 

 in each case. d. I. dorsal lip. 



