No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 363 



as follows : " Es ist ein wunderbares Schauspiel, unter der 

 Loupe diesen plotzlichen Tumult in Dotter Klumpchen zu sehen. 

 Manches Individuum wird von seinen unruhigen Nachbarn 

 eingemal hin und her geschoben bevor es zu Ruhe kommt." 



Newport ('5l) observed the same, and says : " In some ova 

 there is such an unusual displacement of the segments as al- 

 most to prevent the identification of the parts in the subse- 

 quent changes." It thus becomes impossible for any furrow 

 to coincide with a straight line forming the median plane of 

 the embryo, unless after the egg has reached a very late stage, 

 the cells shift back and arrange themselves along definite lines, 

 which seems extremely improbable. 



I believe the only rational conclusion which can be drawn is 

 that no cleavage furrow in these vertebrates bears a fixed rela- 

 tion to the future median plane of the embryo. 



Homology of cleavage furrows. — The supposed homology 

 of the first cleavage furrows throughout the vertebrata is based 

 upon its relation either to the ovic axis or the embryo. Since 

 it bears no constant relation to either, it seems questionable 

 if any homology can be drawn. 



In attempting to homologize the second furrow more serious 

 difficulties are encountered, since we must consider this cleav- 

 age as made up of two entirely independent cell divisions, the 

 plane of which bears no fixed relations to the first furrow, either 

 in time or direction, or to the ovic axis. If we compare the 

 second meridionals in the two eggs of Petromyzon (PI. XIX, 

 Figs. 6 and 8), the difficulties are evident. 



In comparing the Teleostean and Amphibian types of 

 cleavage, Rauber was forced to postulate the loss of the first 

 set of equatorials in the Teleost. H. V. Wilson believes such 

 a loss probable and holds with Rauber that the equatorials in 

 the Teleost are represented by the edge of the blastodisc. 



Agassiz and Whitman think this improbable and accept the 

 homology of the third cleavage in the two groups (according 

 to Rauber and Wilson the third Amphibian and the fourth 

 Teleostean). 



It is evident that I cannot accept either of the above homol- 

 ogies and do not believe that even in the Amphibia we can 



