218 



'87 paper. Moreover, sections are occasionally observed, like Fig. 7, 

 which show an abrupt slit-like continuation of the archenteron into 

 the tongue. Such a slit-like continuation can only be interpreted as 

 due to a very recent separation of the yolk cells. 



In sections the same stages in the formation of the ventral lip 

 are present as in the case of the dorsal lip. A shallow furrow is 

 found, at the bottom of which may be seen intercellular depressions 

 between the outer ends of the cells, which in an earlier stage are 

 closely packed. The outer ends of the cells between which the de- 

 pressions lie, are heavily pigmented, and more or less rounded. It 

 is doubtless true that the same change in shape (contraction), which 

 gives a rounded outer end to these cells, is also responsible for the 

 depressions arising between them. The shallow furrow is succeeded 

 by a sharp one, which appears on the surface as a pigmented line, 

 and this deepens in the well known manner (Fig. 7). 



^ Closure of the blastopore in compressed and in- 

 verted eggs. In the normal uncompressed egg the dorso-ventral 

 diameter of the blastopore, at the time when the ventral lip makes 

 its appearance, is V4 of the egg diameter. In the compressed eggs, 

 the relative diameter of the blastopore seems slightly smaller. In 

 a representative case the diameter of the blastopore and the egg 

 diameter were as 68 to 100, instead of, as in the uncompressed egg, 

 75 to 100. The apparent change in relative size is due to the fact 

 that under compression the egg diameter increases at a greater rate 

 than does the diameter of a minor circle like the blastopore. 



If after the blastopore lip has been outlined in the fashion de- 

 scribed in the preceding section, particular ectoderm and yolk cells 

 are kept under continuous observation i) the following facts may be 

 observed: 1) Yolk cells adjacent to the dorsal lip, gradually dis- 

 appear under that lip ; this disappearance I interpret as due to over- 

 growth on the part of the dorsal lip. 2) Yolk cells adjacent to the 

 ventral lip, disappear in a similar manner under that lip; this dis- 

 appearance is likewise interpreted as due to overgrowth on the part 

 of the ventral lip. 3) Ectoderm cells close to the dorsal lip, and 

 close to the ventral lip, gradually disappear round their respective 

 lips. Such cells, it would seem, must become a part of the arch- 



1) My practice was to locate a cell with reference to a division of 

 the micrometer eye-piece, and then take a good look at it every half- 

 minute, resting, often shutting, the eyes meantimes. Free hand sketches 

 with micrometer measurements were made at frequent intervals. The 

 observations are physically trying. 



