510 BERTRAM G. SMITH 



side of the median notch, which may be shifted toward the median 

 Hne during the process of overgrowth. This point can be definitely 

 settled only by experiment; but in the absence of experimental 

 data we can say that there is no positive evidence of such a proc- 

 ess taking place, while certain considerations weigh against it. 

 For in certain observed cases rapid shifting of material is accom- 

 panied by a roughening of the surface with the formation of par- 

 allel fissures, as in the region just outside of the neural folds 

 during their formation and progress toward the median line. 

 There is an entire absence of any such feature in the dorsal lip 

 of the blastopore. 



A region at the posterior end of the embryo, which is roughly 

 estimated at 16 degrees, is formed through the concrescence of 

 the lateral and ventral lips of the blastopore. A part of this 

 material has been brought through a distance of 60 degrees by 

 the overgrowth of the ventral lip of the blastopore; it will be 

 observed that this distance equals that of the overgrowth of the 

 dorsal lip of the blastopore. 



At the close of Stage 13, when the embryonic body is for the 

 first time clearly indicated, it has a total length of about 148 

 degrees. The posterior end is formed around the vegetal pole; 

 the anterior end hes about 40 degrees from the animal pole. 

 Hence the statement made in Part I (Smith '12) to the effect 

 that the axis of polarity of the late ovarian egg defines the prin- 

 cipal axis of the embryo is not quite accurate; but the embr^'o is 

 formed almost wholly in a hemisphere of the egg lying to one 

 side of the axis of polarity. A review of its history shows that 

 the embryo is formed almost entirely out of material derived 

 from a band of cells lying in the equatorial region of the late 

 blastula and early gastrula, and that this band of cells is narrow 

 on the ventral, broad on the dorsal side of the egg (figs. 116 and 

 123). 



Goodale ('11), after reviewing the literature of the subject 

 in connection with his own work on Spelerpes, concluded that 



The amphibian embryo develops almost entirely in a vertical half 

 of the egg, the tail appearing near the lower pole, while the anterior 

 end of the body develops in greater or less degree in the upper hemisphere, 



