GASTRULATION AND EARLIEST DEVELOPMENT 183 



the shape of a crescent and the border of which (fig. 356*), 

 as further development shows, indicates the limit of t^e 

 head as far as the gill-slits. On this plate two suckers, 

 the mouth and the two olfactory grooves are afterwards 

 found. Further experiments have shown me, that the mark 

 c is generally found a little distance further behind the 

 fold which represents the border of the sense-plate than 

 indicat-d in fig. 2 of my publication of 1916 and that as 

 a consequence we probably can not think of a closer 

 relation between the so-called sense-plate and the apical 

 p'ate of a trochophore as alluded to at that time (p. 9). 



The mark d also does not alter its position in a marked 

 way. It is found again in the region where afterwards the 

 gill-slits will break through. 



The above results were published by me in 1916. Since 

 then I have repeated and completed these experiments and, 

 by using specially sharpened hedgehog quills, have now 

 succeeded in making marks not only at the animal but 

 also at the vegetative pole, i. e. at the opposite point of 

 intersection of the first two cleavage-furrows, where the 

 yolk accumulated here renders marking more difficult since 

 a little lesion causes the yolk to protrude in great quantity. 

 During further development, during cleavage and gastru- 

 lation, the mark at the vegetative pole did not change in 

 the least its position with regard to the one at the animal 

 pole. After the blastopore border had appeared it was 

 found just in front of the ventral lip within the white area 

 and finally it was lying at the place where the blastopore 

 closes to a short slit, still right opposite the mark at the 

 animal pole. This shows that the white endodermic area 

 does not change its position at all while the blastopore 

 border is overgrowing it and while at its periphery the 

 smaller endoderm cells are proliferating and by sinking in 

 and splitting are forming the archenteric cavity 



The results obtained with Rana esculenta and Ambly- 

 stoma tigrinum agree on the whole with those on Rana fusca. 

 The closure of the blastopore, however, does not occur 

 exactly opposite the animal pole as in the latter form. In 

 Rana esculenta this point lies slightly more to the dorsal 

 side, the rudiment of the embryo as a consequence covering 

 less than 180° of the circumference of the egg, whereas in 

 Amblystoma it is situated somewhat more to the ventral 

 side, the base of the embryo extending here dorsally over 



