CH. X] MODIFICATION OF CLEAVAGE 99 



the plates. They are generally more or less parallel to the 

 first furrow. The furrows of the fourth order come in as a 

 rule at right angles to the last furrows, and therefore vary 

 in position according to the position of the third furrows 

 (Fig. 81, C). 



The later development of these eggs is as follows : if the eggs 

 have been much compressed, the blastopore appears always at 

 the periphery of the flattened egg, i.e. at the edge, and in the 

 space between the two plates ; when the eggs are not so much 

 compressed, the blastopore appears near the edge but more or 

 less upon one or the other surface. Curiously enough, just 

 before the closure of the blastopore, its opening is found to lie 

 at the edge of the same side at which it first appeared. Born in- 

 terprets this result as due to a rotation of the whole egg during 

 the closure of the blastopore. The eggs, he believes, are able 

 to rotate in the space between the glass plates around an axis 

 at right angles to the plates. The medullary fold appears also 

 at the edge of the compressed egg. 



3) Eggs compressed laterally and kept icitli the black pole to 

 one side (Fig. 30, C). If the eggs, laterally compressed, are 

 kept after compression in a horizontal position, i.e. with the 

 primary axis horizontal, other phenomena appear. Under these 

 circumstances, Born says that a streaming of the contents of 

 the egg takes place. The cleavage of these eggs corresponds 

 in general to that of the laterally compressed eggs, with nor- 

 mally directed, i.e. vertical, axis. 



4) Eggs compressed between two plates oblique to each other, 

 so that the eggs lie in a wedge-shaped space. The first two fur- 

 rows are at right angles to the compressing plates, which are 

 inclined 12 degrees to each other. The furrows of the third 

 order are in the smaller, dark and more compressed cells at 

 right angles to the plates, while in the yolk-cells, which are little 

 compressed, the furrows are horizontal. The details of these 

 experiments of cleavage have not been worked out by Born so 

 fully as in the cases where the compressing plates were parallel 

 to each other. 



Hertwig ('83, b) has described the first cleavage of one of 

 these eggs compressed by plates inclined 45 degrees to each 

 other. The first cleavage divides a smaller protoplasmic por- 



