48 DEVELOPMENT OF THE FROG'S EGG [Cii. IV 



ing each other at right angles. Prevost and Dumas, in 1824, 

 gave for the first time a definite description of the cleavage of 

 the frog's egg. They described the first furrow beginning in 

 the black hemisphere and stretching out into the white hemi- 

 sphere. They saw, moreover, the small lateral creases or folds 

 along the edges of the first cleavage-furrow. The second fur- 

 row, they said, cuts the first at right angles. When the dark 

 hemisphere is divided into four segments, they saw that then 

 a third equatorial furrow forms near the boundary of the two 

 hemispheres. The next furrows, they said, appear parallel to 

 the first. 



Rusconi ('26) observed that the furrows were not simply sur- 

 face-lines, but cut up the yolk into separate parts, producing 

 finally a large number of small pieces, which he believed were the 

 elements from which the different parts of the body developed. 

 Von Baer's description of the jDrocess, in 1834, is much more 

 exact than the accounts of his predecessors. His interpretation, 

 too, is much clearer and nearer to the truth. He said that the 

 advance of the first furrow into the lower hemisphere goes on 

 as though it were overcoming great difficulty. The tearing 

 apart of the yolk into halves is brought about as a result of a 

 living activity, and the power to divide the ovum does not reside 

 only in the surface of the ovum, but extends throughout the 

 whole mass. Von Baer noticed that after the division the 

 cross-diameter of the egg is greater than the vertical diameter 

 in the proportion of six to five, and he said that the difference 

 would be greater were it not for the egg-membrane. The ten- 

 dency, he said further, is to form two spheres which are, how- 

 ever, compressed against each other by the membrane. Since 

 the division of the white hemisphere progresses more slowly, 

 and since the third division is nearer to the upper hemisphere, 

 we can understand why the dark portions are always smaller 

 than the white portions. When the surface appears again 

 smooth (owing to the smallness of the portions into which the 

 ovum has divided), the egg is very distinctly larger than at 

 first. A^on Baer concluded that material is taken up from the 

 outside to form the albumen, and hence to enlarge the ovum. 

 He interpreted the process of cleavage as the self-division of the 

 individual to form innumerable smaller units. In the later 



1 



