BILATEKALITY IN CRYPTOBRANCHUS 393 



phibian egg is indeterminate in the sense that there is no causal 

 connection between the direction of early cleavage furrows and 

 the median plane of the embryo. This view has gained ground 

 in spite of the fact that in particular species there is an ap- 

 proximate coincidence between either the first or the second 

 cleavage furrow and the median plane. 



In the frog's egg the plane of first cleavage tends to coincide 

 with the median plane of the future animal, though the relation 

 is far from exact (Newport, '54; Roux, '85, '87; Morgan and 

 Boring, '03; Jenkinson, '09, pp. 165-168 and Appendix A). 

 Brachet ('03, '05) demonstrated experimentally that each of the 

 first two blastomeres of the segmenting egg of the frog is capable 

 of producing an entire embryo only when the plane of first cleav- 

 age coincides with the previously determined plane of bilateral 

 symmetry; right and left halves, dorsal and ventral sides, anterior 

 and posterior ends, are predetermined in the undivided egg, and 

 in normal development it makes no difference how the egg is 

 cut up by the early cleavage furrows. McClendon ('09, '10) 

 was able to remove completely one of the first two blastomeres 

 of the egg of the tree-frog Chorophilus triseriatus. A large 

 number of eggs were thus operated upon; in a considerable 

 number of cases the remaining isolated blastomere gave rise to a 

 complete normal embryo, and some of these lived to the larval 

 stage. These results, considered in connection with the findings 

 of other investigators, led this author to conclude that each of the 

 first two blastomeres is totipotent only when the first cleavage 

 furrow bisects the gray crescent. 



In the newt Diemyctylus viridescens, Jordan ('93) found that 

 in the majority of cases the first cleavage furrow forms at right 

 angles to the direction of the future median plane. Jordan 

 does not interpret this relation to mean that there is any causal 

 nexus between the two. Spemann ('01-'03) found that in the 

 newt Triton cristatus, the first furrow is usually (two-thirds to 

 three-fourths of all cases) at right angles to the sagittal plane, 

 and separates the material for the dorsal and ventral halves of 

 the embryo; only occasionally (one-fourth to one-third of all 

 cases) do sagittal plane and first furrow coincide. Herlitzka 



