80 DEVELOPMENT OF THE FROG'S EGG [Cn. VII 



showing spina bifida is owing to the differentiation in situ of cells 

 that in the normal embryo are first carried to the dorsal surface 

 before they differentiate into their definitive organs. 



Roux first pointed out that the embryo described by him 

 showed that the material for the two sides of the embryo is 

 laid down in a ring, and that by the growing together (con- 

 crescence) of this ring along the mid-dorsal line of the embryo, 

 the two halves of the body are brought together. The same 

 method of formation of the embryo by concrescence has been 

 described as taking place in other vertebrate embryos, and cer- 

 tain writers have even affirmed that this is the method by which 

 all embryos of vertebrates are formed. In the main, Roux's 

 conclusion for the frog seems to be correct, 1 but in one respect 

 not an unimportant exception must be taken to his statement. 

 If the material be laid down as a ring of tissue around the equa- 

 tor, and if, by its coining together (apposition), the two halves 

 of the embryo result, it follows that the embryo should be at 

 least as long as one semicircle of the surface of the egg. 

 Further, we have seen that the anterior end of the medullary 

 plate lies somewhat above the point of appearance of the dorsal 

 lip of the blastopore, so that the embryo would be, on Roux's 

 supposition, even longer than a semicircle. But if we measure 

 the medullary plate of the embryo at the time of its first appear- 

 ance, we find that in length it is only about one-third of the 

 length of the circumference of the egg. It follows, then, that 

 as the material comes to the mid-dorsal line in the normal 

 embryo, it must also become more concentrated, so that the 

 length of the medullary plate is less than the length of the 

 material of its halves. There is an accrescence or concentration 

 of material combined ivith a concrescence or union of material 

 from the two sides. 



1 Although Roux did not foresee the possibility that material might grow 

 around the equator from the dorsal lip of the blastopore, my own experiments 

 show, I think, that such a transfer does not take place. 



