396 BERTRAM G. SMITH 



dorsoventral differentiation appears at different stages in different 

 species of animals. 



Schultze ('00) was probably the first to describe the bilateral 

 organization of the late blastula of the frog, and Ishikawa ('08, 

 '09) has described a similar condition in the developing egg of 

 the giant salamander of Japan. 



In the segmenting egg of the frog, Bellamy ('19) has demon- 

 strated a primary area of high susceptibility to the action of 

 reagents, in a meridian that bisects the gray crescent and near 

 the center of the pigmented hemisphere; also a secondary area of 

 high susceptibility in the equatorial region immediately above the 

 gray crescent, hence just above the site of the dorsal lip of the 

 future blastopore. 



E. Embryonic axes in relation to bilateral symmetry. In the 

 ovarian egg of Crypt obranchus, the first visible differentiation 

 having reference to the form of the adult is manifested in what we 

 call polarity: an active pole, rich in cytoplasm, is differentiated 

 from an opposite and relatively inactive pole concerned mainly 

 with the storage of food materials. At this stage of develop- 

 ment the structure in any plane taken at right angles to the polar 

 axis is radially symmetrical. Eventually, this polar axis deter- 

 mines approximately the principal axis of the embryo, or the 

 axis of anteroposterior differentiation. 



The further step necessary for the estabUshment of bilateral 

 symmetry is the appearance of a secondary axis, the dorsoventral 

 axis, extending at right angles to the first. In the egg of Crypto- 

 branchus this secondary axis becomes apparent through the 

 unequal development of opposite sides of the roof of the blasto- 

 coele; the region of more active cell division becomes the dorsal 

 side of the embryo. 



It is perfectly obvious that these two axes supply all the differ- 

 entiations necessary to establish a condition of bilateral symme- 

 try, for the plane determined by the intersection of these two axes 

 divides the egg into halves which were originally ahke, and which 

 are modified in a corresponding manner by the differentiation 

 that proceeds along the secondary axis. 



