Cii. V] EARLY DEVELOPMENT OF THE EMBRYO 51 



will be found that changes take place at this time in the blasto- 

 poric region with great rapidity. 



Pfluger has described ('83) these changes, and we may 

 follow his admirable account, subsequently adding other facts 

 that have since been discovered. The eggs which Pfluger 

 studied 1 were taken from the uterus at twelve o'clock midday, 

 and placed in a row on a thick glass mirror. The mirror was 

 then put into a dish, and water added to the depth of 2 mm. 

 In this way, owing to the reflection of the lower pole by the 

 mirror, both hemispheres of the egg could be watched. Dur- 

 ing the night, when the temperature was low, the eggs de- 

 veloped more slowly. At six o'clock in the morning the 

 thermometer stood at 16° C. At this time the eggs showed 

 on the lower hemisphere, and in the upper fourth of that region 

 and therefore just beneath the equator, the first trace of the 

 dorsal lip of the blastopore. By ten a.m. the long horizontal 

 split (dorsal lip of the blastopore) had become distinctly marked 

 as an indentation of the surface of the egg. At eleven a.m., 

 the dorsal lip had moved somewhat further below the equator 

 of the egg, i.e. toward the lower pole. The " split" is now 

 broader, and its corners turned down so that it forms a cres- 

 cent, with the lower pole of the egg-axis as its middle point. 

 The diameter of the crescent is to the egg-diameter as 2:3. 

 From the corners of the crescent a furrow continues to extend 

 on each side around the white hemisphere. The progress of 

 the dorsal lip toward the lower pole is not due to a rotation 

 of the egg as a whole, but to the migration of the dorsal lip over 

 the white hemisphere. At half -past twelve o'clock (twenty- 

 four hours after fertilization), the dorsal lip has progressed 

 further toward the lower pole. The crescent has at the same 

 time extended so as to form a half-circle whose diameter is 

 somewhat less than in the preceding stage. It stands now in 

 relation to the diameter of the egg as 1 : 2. 



By one o'clock p.m., the semicircle forming the dorsal and 

 lateral lips of the blastopore has extended so as to form a 

 complete circle (Fig. 19, A, IV). The white yolk-cells pro- 

 trude from the centre of this circle and form the so-called 



1 Bufo cinereus. 



