472 BERTRAM G. SMITH 



less the mechanical result of the location of the earlier course of 

 the third furrows nearer to the first than to the second; they 

 swerve from the vertical toward the nearest existing cleavage 

 furrow. An analogous pattern may sometimes be observed in 

 the cracking of the corners of a section of cement walk. 



I have observed this biradial cleavage pattern in corresponding 

 stages of the lower hemisphere of occasional eggs of Necturus; 

 it is clearly expressed in the cleavage of Desmognathus as figured 

 by Wilder ('04) and Hilton ('04 and '09). 



The same tendency to join the nearest existing vertical furrow 

 is shown by those fourth cleavage furrows, as a rule not yet com- 

 plete, that come in vertically. They usually join the second 

 furrow, at a much greater distance from the lower pole than the 

 intersection of the third with the first. 



In the vicinity of the vegetal pole, both first and second cleav- 

 age furrows are now only faintly expressed. 



In about half the eggs of this stage cell division has proceeded 

 a little more rapidly on one side of the egg than on the other; 

 the cells are smaller in surface view, more numerous, and the 

 cleavage furrows are more uniformly complete (figs. 87, 88 and 

 210). Thus there is an excentric development of the blasto- 

 disc, whereby a condition of bilateral symmetry in the cleavage 

 pattern is produced. This excentric development is a more 

 constant feature in the stages immediately following. The 

 question naturally arises whether this bilateral symmetry in 

 the cleavage pattern has any morphogenetic significance: is it 

 the outward expression of the establishment of the permanent 

 bilateral symmetry and antero-posterior differentiation of the 

 embryo? In other words, does the axis of bilateral symmetry 

 in the cleavage pattern fall in the median plane of the future 

 embryo? This question will be considered in a later paper, in 

 connection with ^le study of the internal development. 



Such an excentric development of the blastodisc has been 

 described for Amblystoma and Necturus by Eycleshymer ('95 

 and '04), who cites similar observations on other vertebrates 

 by various writers'. Eycleshymer speaks of a "second area of 

 accelerated cell division" as distinguished from the primary area 



