i82 THE ANCESTRY OF VERTEBRATES 



first two cleavage planes) is a little shorter when measured 

 dorsally than when measured ventrally (the two ventral 

 cells in stage 4 being slightly larger than the two doisal 

 ones), it follows that the closure of the blastopore takes 

 place exactly at the vegetative pole. 



If now we consider the eggs marked at b, c, or d, which are 

 the points of intersection' on the third or equatorial cleavage 

 furrow, we find that these marks during the gastrulation process 

 remain almost stationary. Their distance from the mark 

 a at the animal pole certainly increases, but the difference 

 is only very slight. When the dorsal blastopore lip is 

 formed, the mark at b is found lying about just as far in 

 front of it as in the eight-cejled stage the point b is distant 

 from the egg equator. The more the blastopore lip is then 

 shifted backwards the greater the distance becomes. Finally 

 the mark is found on the medullary pKte exactly behind 

 the cerebral plate. From this it is quite evident that there 

 can be no question about the whole rudiment of the embryo 

 being formed by concrescense of the lateral borders of 

 the blastopore, since the greater part originates in situ in 

 front of the dorsal rim. The rudiment of the cerebral 

 plate is situated from the beginning between the marks a 

 and b. That there would be an invagination of animal 

 cells round the dorsal blastopore border to invest the 

 archenteron roof is rendered improbable by other pricking 

 experiments in which marks were made slightly in front 

 of the crescentic border as it was just appearing. In some 

 cases truly I found this mark to approach and to reach 

 the blastopore border, but soon the eggs on which 1 worked, 

 which weie of Rana esculenta, proved to develop abnor- 

 mally and to show a tendency to "spina bifida," i e. a 

 retarded closure of the blastopore. In these eggs the dorsal 

 blastopore border, instead of moving backwards, moves 

 forward and in this case an invagination of cells round 

 it seems to take place. In normally developing, eggs, 

 however, a mark, however closely it lies in front of the dorsal 

 blastopore border, does not approach the latter, which 

 shows that there occurs no invagination of cells from the 

 surface, as postulated by LWOFF. 



The mark at c is found again some distance before the 

 anterior end of the cerebral plate, in front of (properly 

 speaking behind) the border of the so-called sense-plate 

 which lies round the front part of the cerebral plate in 



