358 BERTRAM G. SMITH 



Discussion 390 



A. Bilateral organization previous to fertilization 390 



B. Influence of the spermatozoon and of environmental factors, 391 



C. Relation of the first cleavage furrow to the median plane of the embryo. 392 



D. The bilateral symmetry of the blastula 394 



E. Embryonic axes in relation to bilateral symmetry 396 



Summary 397 



Bibliography 398 



ANALYSIS OF THE PROBLEM 



In the higher animals, the most obvious features of organiza- 

 tion are those expressed by the terms bilateral symmetry, antero- 

 posterior differentiation, and dorsoventral differentiation. Since 

 bilateral symmetry is always accompanied by the other features 

 mentioned, the expression bilaterality is alone sufficient to desig- 

 nate the type of structure under consideration. 



Bilaterality is a feature of such fundamental importance in the 

 organization of the embryo that we should expect it to appear 

 very early in the ontogeny, and the problem of tracing its origin 

 is one of considerable interest in relation to theories of heredity 

 and development. In particular one desires to know the earliest 

 manner of expression of the definitive bilateral symmetry of the 

 embryo, and whether the hereditary factors that are undoubtedly 

 at work can be modified by external influences. 



In a previous communication (Smith, '12, II) it has been shown 

 that the polarity of the late ovarian egg of Cryptobranchus 

 allegheniensis, as expressed by its telolecithal character, es- 

 tablishes approximately the principal axis of the embryo: the 

 anterior end forms about 40° from the animal pole, the posterior 

 end quite accurately at the vegetal pole. But a careful study of 

 the ovogenesis recorded in another paper (Smith, '12, I) has not 

 revealed any feature of the ovarian egg that enables us to dis- 

 tinguish right and left, dorsal and ventral surfaces; in the mature 

 but unfertilized egg the condition is one of radial symmetry 

 and axial differentiation, with no trace of bilateral symmetry. 

 Subsequent observations, made by examining in toto preparations 

 of the ovarian eggs cleared in various oils, have not altered this 

 conclusion. Even the polarity of the egg is not visibly expressed 



