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the egg, and the distance between the periphery of the projection 

 image of the egg and the lip at successive moments. It may thus 

 be found that tlie dorsal Hp makes its appearance 15 "^ (or close to 

 that angle) below the equator of the egg, and travels 25" or there- 

 abouts over the yolk, before the ventral lip develops. In the same 

 way, the diameter of the blastopore, after its entire lip has been 

 outlined as an edge, may be measured at successive moments, and 

 its angular measurement computed. It is found that when the whole 

 lip is first outlined as a distinct edge (i. e. when the ventral part of 

 the lip has just been differentiated), the angular measurement of the 

 Itlastopore is close to 95°, and its diameter is about ^U of the egg, 

 diameter. The blastopore continues to occupy the lower pole of the 

 egg until its angular measurement is about 40", its diameter meas- 

 uring Vs to V4 of the egg diameter. By the end of the 90° ro- 

 tation, the angular measurement of the blastopore is close to 25", 

 its diameter being about V5 that of the egg. It will be seen that 

 the angular measurement of the distance lying between the points 

 where the dorsal and ventral lips respectively make their appearance, 

 is 120" (95 " + 25") — that is, apart of the white surface, measuring 

 120", is ultimately covered by the contracting blastopore lip. 



The size of the blastopore at successive periods has not always 

 been correctly given. Thus Roux ('88) would have it that 180" inter- 

 vene between the dorsal and ventral lips. And while Morgan ('93, 

 '97) is correct, according to my observations, in believing that the 

 greatest distance between the lips is about 120", he is in error in 

 supposing that the ventral lip is so late in developing. Morgan 

 represents the blastopore, at the time when the ventral lip first 

 appears as a distinct edge, as having narrowed to about V4 the dia- 

 meter of the egg, whereas (in Chorophilus) this part of the lip 

 i s d e V e 1 p e d a n d c a p a b 1 e f V e r g r w t h w h e n the blasto- 

 pore diameter is ^j^ that of the egg. 



The length of the neural plate when first differentiated is a fact 

 of importance, in considering how the frog embryo is formed from 

 the gastrula. I agree with Morgan in placing the original length 

 of the plate at 120". In Chorophilus the differentiation of the neural 

 plate proceeds as follows. Shortly after the 90" rotation, the upper 

 (dorsal) surface of the embryo becomes somewhat flattened, the flatten- 

 ing progressing forwards from the blastopore. The flattened area is 

 the connnencing neural plate. Round the edge of this area pigment 

 accumulates — the area itself is light in color. The area is least 

 distinctly marked off at its anterior end. When the anterior end 



