Ch. YII] PRODUCTIOX OF ABXORMAL EMBRYOS 79 



It is important to know detinitelj the origin of the material 

 that forms the equatorial ring. We have seen that the ring 

 appears at the same time that the bhistopore-lips extend around 

 the equator of the egg. Does this material also extend out 

 laterally from the dorsal lip of the blastopore along the sides, 

 or is the material already present as a circular ring of tissue, 

 from which the lips of the blastopore differentiate ? A study 

 of the normal embryo combined with experiments gives, I 

 believe, a conclusive answer to these questions. In the first 

 place, if the dorsal Up be entirely destroyed, so that it cannot 

 advance, nevertheless the lateral lips still a2)pear and extend 

 backward. If a point of the surface be injured just in front of 

 one (or both) of the advancing corners of the dorso-lateral lips^ 

 the advance of the latter would be stoj^ped if an actual transfer 

 of material were taking place; nevertheless, on the posterior side 

 of the point of injury, a depression of the surface, marking the 

 blastoporic rim, appears, and continues to extend backward. 

 The same thing happens if injuries be made at two consecutive 

 points in the direction of extension of the lateral lip. Now if 

 material were actually transferred backward from the dorsal 

 lip and around the equator of the egg, its movement would be 

 stopped when the dorsal lip M'as seriously injured, so that the 

 lateral lips of the blastopore, and, later, the medullary folds, 

 would not appear, or else their appearance would be delayed. 

 Further, if there were, in reality, any such transfer backward 

 of material around the equator, its progress would be stoj)ped 

 when the material reached the points of injury made along the 

 line of the lateral lip. On the contrary, the appearance of the 

 lateral lips, after the destruction of the dorsal lip, takes place 

 as though no hindrance were present. 



The experiments point clearly to the conclusion that tliere is 

 no backward transfer of building material, but that the mate- 

 rial for the dorsal surface is already present as a ring around 

 or near tlie equator of the egg. 



If the normal embrj- o be studied by means of sections at the 

 period of the extension of the lateral lips of the blastopore, 

 the material of the ring is found to be already present in the 

 region into which the lateral lips extend. The evidence from 

 these various sources proves that the j^^'oduction of the e7nbryos 



