No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 365 



The gradual obliteration of the distinct periods of rest, and 

 the consequent transition to a condition of constant activity is 

 brought about at an early stage. It is of interest when com- 

 pared with the regularity manifested in the cleavage of certain 

 Teleosts (Serranus, Wilson, '91). 



The transition is due to the early appearance of areas of 

 special activity. While the region of greatest activity is un- 

 doubtedly at the superior pole, there is another portion of the 

 blastoderm in which cell division becomes accelerated ; this is 

 that portion which is most deeply pigmented and which is 

 destined to lie somewhere in the future tail region of the 

 embryo. 



Kolliker ('79) observed these differences in the embryonic 

 area of the chick. I quote his words: "Die Furchung geht 

 immer asymmetrisch vor sich, so dass ohne Ausnahme die eine 

 Halfte der Keimscheibe in der Zerkluftung der andern voran 

 ist . . . der schneller sich furchende Theil zum spateren hin- 

 teren Theil des Blastoderma sich gestaltet, in dem die ersten 

 Spuren des Embryo entstehen." 



In the blastoderm of Amphioxus Lwoff ('92) describes an 

 area of accelerated cleavage at the dorsal lip of the blastopore : 

 " die Zellenvermehrung nicht iiberall gleichmassig vor sich 

 geht, sondern sich vorzugsweise an einer Seite konzentriert, 

 die zur Dorsalseite der Gastrula wird." 



The fact that areas of accelerated activity appear in early 

 cleavage stages is, as we shall see, one of fundamental impor- 

 tance in orienting the embryo. 



If we compare the variations observed in the three types 

 studied, it is evident that the greatest variation is in the 

 Teleost while the least is in Amblystoma and Rana. The 

 order of variation might be expressed thus : the first and 

 second furrows in Coregonus show the irregularities found in 

 the second and third of Petromyzon, while the second and third 

 of Petromyzon show about the same degree of variation as the 

 third and fourth of Amblystoma. 



From the preceding pages we conclude that in Petromyzon, 

 Amphibia, and Teleost the more detailed the study of cleavage 

 the more apparent become the irregularities, and we wonder 



