235 



In the third cleavage the grooves rarely pass in horizontal planes 

 as they generally do in the Anura and most of the Urodela. The 

 nearest approach to the horizontal position is that shown in P'ig. 30 

 in which they occupy a position intermediate between meridional and 

 horizontal. In most eggs the cleavage grooves are irregularly formed 

 and it might be said that the variations are so numerous and so di- 

 verse that a special description must be written for each egg. 



It is scarcely necessary to state that if these cleavage planes mark 

 embryonic areas, the amount of material set apart in different eggs for 

 similar parts of their respective embryos, must be exceedingly variable, 

 and these excesses and deficiencies must be corrected by a corres- 

 ponding retarded or accelerated growth until the norm is reached, but 

 there is not the slightest evidence that such corrections occur. 



These wide variations have been repeatedly observed not only in 

 various amphibia but also in practically all classes of vertebrates: in 

 Amphioxus by Wilson ; in Petromyzon by McCluke, Kupffer, Eycle- 

 shtmer; in Dipnoans by Semon; in Ganoids by Salensky, Dean, 

 Whitman and Eycleshymer ; in Teleosts by Coste, Hoffmann, His, 

 Agassiz and Whitman, Kingsley and Conn, Clapp, Sobotta and 

 others; in Reptiles by Agassiz and Clark, Oppel, Sarasin; in Aves 

 by Coste, Koelliker, Kionka ; in Mammals by Duval, Van Beneden, 

 Assheton, Sobotta and many others. 



Shifting of Cells during Cleavage. 



A pronounced characteristic of cleavage which has hitherto re- 

 ceived but little consideration, is the torsion or twisting of cleavage 

 grooves due to the displacement of cells. It was long ago described by 

 VON Baer in these words: "Es ist ein wunderbares Schauspiel, unter 

 der Lupe diesen plötzlichen Tumult in Dotterklümpchen zu sehen. 

 Manches Individuum wird von seinen unruhigen Nachbarn einigemal 

 hin und her geschoben, bevor es zur Ruhe kommt." Newport ob- 

 served the same and says: "In some ova there is such a displacement 

 of the segments as almost to prevent the identification of the parts 

 in the subsequent changes". 



The important bearing of this shifting upon the question under 

 discussion was pointed out by Hertwig, viz: "Endlich sei noch auf- 

 merksam gemacht, was Roux bisher auch immer unberücksichtigt ge- 

 lassen hat, daß an vielen Eiern die beiden ersten Teilebenen bald 

 nach ihrer Entstehung ihre Form und Stellung zu einander vollständig 

 verändern". Jordan and Eycleshymer concluded from studies on 

 Amblystoma, Diemyctylus, Rana and Bufo "that cells originally to one 



