364 EYCLESHYMER. [Vol. X. 



speak of the third cleavages as homologous. The difficulty lies 

 in the fact that this so-called furrow is made up of four dis- 

 tinct furrows. As already shown, one, two, three, or four of 

 these may pass in equatorial or in vertical planes and we are 

 obliged to say, if we wish to homologize, that three-fourths of 

 this cleavage in one q.^% is homologous with one-fourth in 

 another, etc. 



It is, of course, true that occasionally variations in the 

 Teleost and the Frog correspond very closely, but to suppose 

 that certain variations in the Teleost which happen to corre- 

 spond to variations in the Amphibian are atavistic, as held by 

 some, seems to me extremely hypothetical. The variations in 

 Petromyzon and Coregonus are so frequent that one may say 

 without exaggeration, that very rarely, if ever, at this stage 

 the cleavages in two eggs of the same lot bear precisely the 

 same relation to the ovic axis or the preceding furrows, to say 

 nothing of the disparities in time and point of origin, direction, 

 and rate of progress. 



Jordan and myself repeatedly followed the cleavage of various 

 Amphibia with the hope of discovering a law, but all attempts 

 in this direction proved fruitless. It is even doubtful, as 

 Mauro Rusconi stated in 1836, if the geometrical progres- 

 sion 2, 4, 8, 16, 32, 64, etc., formulated by v. Baer, ever 

 occurs. 



In the egg of Amblystoma the alternating periods of repose 

 and activity are well marked in the early cleavage stages, but 

 as development proceeds this synchronism gradually disappears. 

 The differences of time existing in the periods of activity as 

 indicated by the origins of the furrows may be tabulated as 

 follows : 



1st Cleavage, interval between appearance of first and last furrow ca. — min. 

 2d " " " " ' 1-2 " 



3d " " " " " " " " 4 



4th " " " " " " " " 4 



5th " " " " " " " " 5 



6th " " " " " " " "15 



7th " " " " " " " " 20 



8th " " " " " '• " " 27-35 " 



c)th " " " " " " " " 40-50 " 



