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Chas. B. Wilson 



meet along the middle of the slit and fuse, leaving a small circular 

 orifice at either end. The anterior of these openings is the neuro- 

 pore, the posterior becomes the permanent anus. In embryos 

 developed in salt solution there is at first a similar uniform con- 

 vergence of cells from all parts of the blastopore margin, but this 

 is quickly stopped. There is no rapid growth of the cells at the 

 sides, and consequently no elongation of the blastopore. This 

 cannot be explained by the influence of the salt solution, for if that 

 had any effect at all, it would increase the difference between the 

 activity of the cells at the sides and those at the ends of the blasto- 

 pore i. e. it would retard the more active side cells less than the 

 comparatively sluggish cells at the ends. 



There is also positive evidence in later development that the 

 side cells retain their activity. We are compelled, therefore, to look 

 elsewhere for our explanation. If we examine a median longitudinal 

 section of one of these salt embryos at this period we find that the 

 archenteron is of about the same depth throughout its entire length 

 (fig. 18). The cells of the yolk plug have not withdrawn from the 

 blastopore, and hence do not fill up the posterior part of the 

 archenteron. 



The dorsal lip of the blastopore is larger, thicker, and rounded 

 inward more than normally. This is due to a combination of circum- 

 stances. 



In the first place the lip is hindered in its downward growth 

 by the non-withdrawal of the yolk plug. As it elongates, therefore, 

 it would tend to enlarge at the end, to round inward, and probably 

 to lift up the dorsal surface of the embryo somewhat. 



A second reason is found in the fact just mentioned, that the 

 sides of the blastopore border cannot move inward to meet along 

 the mid-line. That they tend to do so normally shows that they 

 must possess considerable activity. In the present instance some 

 other mode of exercise must be found for this activity. It is found 

 in a migration of cells from the lateral borders toward the dorsal 

 mid-line, and a consequent accumulation of material at that point. 

 This contributes to the enlargement of the dorsal lip. 



3. That there is such a migration of cells, and that the closing 

 of the blastopore is here due not to a backward extension of all 

 the material of the dorsal lip over the yolk, but only of its central 

 portion, as Morgan has suggested for the development of Kana, is 

 clearly shown from the following considerations. 



