METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. I I 



the disintegration of the neural tube. The entodermal layer of 

 the anterior intestinal portal is always a region of high sus- 

 ceptibility and from the portal the disintegration extends an- 

 teriorly along the entoderm of the blastoderm. The foregut 

 could not be distinctly observed. 



16. Relative Susceptibility at Different Ages. While this matter 

 was not subjected to definite experimental test, the much greater 

 susceptibility of the younger stages was so obvious that it 

 could not escape attention. A primitive streak stage will dis- 

 integrate in a few minutes in a concentration of reagent in which 

 a two-day chick will not undergo complete disintegration in 

 two hours. 



//. General Summary of the Gradients in Chick Embryos. In 

 the early stages primitive streak and head fold stages a simple 

 primary antero-posterior gradient is present, disintegration be- 

 ginning at the anterior end of the embryonic axis and proceeding 

 posteriorly. At the time of the differentiation of the medullary 

 plate and folds a double gradient appears and persists through 

 all of the stages that were investigated (through the third day). 

 In general this double gradient is as follows: there are two 

 regions of high susceptibility, the differentiating region of the 

 primitive streak, and the anterior end of the neural axis; from 

 these two regions disintegration proceeds anteriorly and pos- 

 teriorly, respectively, towards the middle of the embryo. The 

 differentiating region of the primitive streak is the most sus- 

 ceptible part of the embryo up to about the third day of incu- 

 bation at which time it gradually decreases in activity in corre- 

 lation with the completion of the somites. The double gradient 

 involves both ectodermal and mesodermal structures; and prob- 

 ably also to at least some extent, entodermal structures. In 

 earlier stages disintegration in the mesoderm proceeds from the 

 segmental plate forward along the somites; but after eight or 

 nine somites have appeared, the anterior and posterior somites 

 are both regions of high susceptibility and from them dis- 

 integration proceeds backward and forward respectively towards 

 the middle. The neural tube is usually more susceptible than 

 the mesoderm of the same level. There is also present a medio- 

 lateral gradient in susceptibility from the neural tube towards 

 the lateral plate. 



