180 



THE ANCESTRY OF VERTEBRATES 



between the other forms as regards the closure of the 

 blastopore. The results of my experiments for Rana fusca 

 have been combined in a single figure, composed from 

 many other figures, two drawings being each time super- 

 posed and held up to the light, the details of one figure 

 being in this way transferred to the other. 



The eggs marked at the animal pole teach us what 

 follows (cf. fig. 35) : , 



The dorsal blastoporic lip is foimed very little below 

 the equator (much less than 25° or 30°, see above) and 



a 



* 



Fig. 35. Results of pricking experiments on the frog egg (Rana fusca). 

 a eight-celled stage seen from the side, b stage with me- 

 dullary plate. 



a, b, c, d, e, situation of the marks at the points of intersection 

 of the first three cleavage furrows. 



1, 2, 3, 4, anterior border of the blastopore in successive 

 stages of contraction. 

 br. rudiment of the gills, 



* (fig. a) limit of black and white area, 



* (fig. b) edge of the so-called sense-plate. 



immediately begins to grow over towards the vegetative side. 

 The ventral blastopore lip is formed about diametrically 

 opposite the animal pole, slightly more to the dorsal side. 

 This border practically does not move, so- that the closing 

 of the blastopore finally takes place at this same point. 

 From this it follows that the dorsal blastopore lip progresses 

 through a little less than 90° (estimated not under 80°). 



When the blastopore has finally narrowed to a short slit 

 and the medullary folds arise, this slit consequently still lies 

 almost diametrically opposite the animal pole which is situat- 

 ed exactly in front of the cerebral plate. The distance 



