GASTRULATION AND EARLIEST DEVELOPMENT 185 



beneath the equator, moves backwards over a considerable 

 distance, while it becomes deeper only quite gradually and 

 extends laterally into a crescent. When it finally closes to 

 a ring it has certainly already travelled over a distance of 

 some 60**. The crescent and the ring into which it passes 

 have here only a very small diameter which I do not believe 

 to be more than 30°. During its contraction the dorsal lip 

 moves considerably faster than the ventral one and after 

 the blastopore has reached the slit-like stage the backward 

 movement of this slit with regard to the animal pole 

 still continues. Thus the dorsal rudiment of the embryo 

 covers hce considerably more than 180**, the ventral side 

 being much shorter. 



Interpretation of the results. — What conclusions may 

 now be drawn from the facts recorded and how are these 

 to be interpreted? In the first place we may state that, 

 since more than half of the base of the embryo is situated 

 in front of the place where the dorsal blastopore lip first 

 appears, there can be no question about the whole embryo 

 being formed by concrescence of the lateral blastopore 

 borders. For the assumption that concrescence would play 

 a more or less important role in the closure of the blasto- 

 pore which gives rise to the posterior lesser half of the 

 embryo, there is not the slightest evidence. It is quite 

 true that in the amphibian egg a fme median line is often 

 seen running from the blastopore forward, which strongly 

 suggests a concrescence suture. Only, as ROBINSON and 

 ASSHETON (1891; remark, this lin^ continues to the fore- 

 end of the cerebral plate where the blastopore has never 

 been! I shall now propose the explanation which seems 

 to me to follow from my theory. Two circumstances give 

 a peculiar character to the gastrulation of Vertebrates: 



1. the white area indicating the future endoderm is situated 

 not opposite the animal pole, as we have good reason 

 to assume must have been the case in radiate ances- 

 tors, but shifted more to the future dorsal side, 



2. the contraction of the blastopore border occurs in a 

 caudad eccentric direction, so that the closure finally 

 takes place nearly opposite the animal pole. 



Comparison with Annelids. — As regards the former phe- 

 nomenon, the displacement of the white endoderm area is 

 fairly considerable, as shown e.g. by fig. 35. This same 

 displacement of the endoderm area, here to the ventral 



