476 BERTRAM G. SMITH 



libriiini, discordant factors are more likely to be present to 

 obscure the normal expression of the course of development. 



The sixth cleavage furrows of the outer set, when latitudinal, 

 divide the macromeres very unequally, cutting off additional 

 micromeres. The number of micromeres, and the extent of the 

 blastodisc, is increased by such latitudinal divisions; the number 

 of macromeres is increased by the sixth cleavage furrows only 

 when these come in vertically. 



In a few eggs, as the one shown in figure 99, there is a marked 

 tendency for the sixth cleavage furrows to come in vertically. 

 Here, as noted in an earlier stage, the embryo seems to be oscil- 

 lating between two possible modes of cleavage; but the tendency 

 to preserve the regular alternation of cleavage furrows is usually 

 the stronger. 



The most marked tendency to vary from the regular pattern 

 of cleavage occurs along the line of excentric development of the 

 blastodisc (figs. 98 and 212), as described under Stage 5. The 

 majority of eggs exhibit this tendency in some degree. 



We have then, in the cleavage pattern of this stage, two tend- 

 encies toward differentiation of the blastodisc: (a) an acceler- 

 ated cell division in the marginal portion, pointing toward the 

 formation of the germ ring; and (b) an accelerated cell division 

 about a radius of the blastodisc, giving a condition of bilateral 

 symmetry. 



DeBussy's figure ('05, fig. 10) representing the blastodisc of 

 an embryo of Cryptobranchus japonicus with forty micromeres 

 strongly suggests excentric development; on one side of the first 

 cleavage furrow only three cleavage furrows reach the equator, 

 on the other side nine. But the author remarks (p. 530) that 

 he has observed no secondary center of accelerated cell division 

 such as has been described by Eycleshymer for Necturus. 



A comparison with earlier stages shows that there is an increas- 

 ing tendency for the micromeres, following a familiar law of 

 developmental mechanics, to lose their original quadrangular or 

 triangular outline and become hexagonal. 



In the living egg, the roof of the segmentation cavity is some- 

 what translucent, and spaces communicating with the cavity 



