No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 353 



views indicate an acceleration of cleavage in the least pig- 

 mented portion, yet sections (Fig, 21) show the region 

 possessing most pigment to be that in which cell activity is 

 greatest. 



After the cleavage shown in Fig. 12 it is impossible to follow 

 the divisions, even in areas ; but by magnifying the egg some 

 30 diameters individual cells may be observed. At this time 

 the cleavage of any particular cell occurs at intervals varying 

 from I hr. to i hr. and 45 min. 



Fig. 22 represents a vertical section through the more and 

 less deeply pigmented portions. In the more deeply pigmented 

 portion the roof of the segmentation cavity is several layers 

 of cells thick, while in the less deeply pigmented portion it 

 has remained a single layer. The relation which the more 

 deeply pigmented portion bears to the blastopore and to the 

 future embryo will be discussed presently. 



Experiments on cleavage. — Hertwig's ('93) recent experi- 

 ments on the Frog and Triton, in which the second furrow 

 was made to pass in a horizontal direction, and in which the 

 path of the succeeding furrows bore a definite relation to the 

 direction and degree of compression exerted, indicate that 

 the planes of cleavage may be entirely changed through the 

 effects of mechanical pressure. 



Experiments of the same kind were made on the eggs of 

 Amblystoma tigrinum in April, 1893, in the Morphological 

 Laboratory of the University of Michigan. Unsegmented eggs, 

 from which the envelopes had been removed, were placed 

 between the cut edges of two glass slides and laterally com- 

 pressed to one-half their equatorial diameter. In order to 

 observe botji poles of the egg, the experiments were carried 

 on in glass dishes placed upon a mirror. 



The first vertical in the 34 eggs examined showed no con- 

 stant relation to the compressed surfaces, in 7 passing through 

 the longest equatorial diameter ; in 9, through the shortest ; 

 and in 18, between the two. The second verticals showed no 

 greater variation than under normal conditions. The follow- 

 ing cleavage was generally equatorial, yet in some cases, 3 out 

 of 34, these furrows were vertical. Beyond this cleavage many 



