230 RICHARD ASSHETON. 



The Secondary Area of Cell Production. 

 I think that every one will agree that after the closure of the 

 blastopore, the embryo grows in length by the proliferation of 

 cells at the spot which formerly formed part of the lips of the 

 blastopore. 



There is very little doubt that rapid growth at this spot takes 

 place before the final closure of the blastopore ; the question is, 

 when does this growth begin ? 



Again, it must be remembered that growth of the embryo as 

 a whole, derived from the rapid multiplication of the cells in this 

 area, is growth in length. It is the secondary area of growth 

 comparable to the secondary area of growth or primitive streak 

 of the rabbit. 



If there is such a growth backwards of the blastoporic lips 

 before their closure, there will then be a portion of the future 

 gut cavity of the embryo that will have been formed, not by a 

 splitting nor by an invagination, but by a growth backwards of 

 the blastoporic lips. 



Amphioxus, after the completion of the process of invagina- 

 tion, begins to grow in length. According to Hatschek's 

 account this was largely due to the activity of two pole cells. 

 Recently, however, Wilson has stated very clearly that these 

 pole cells are " a myth." They never exist at any time, but 

 the posterior region of the larva of later stages " is rapidly 

 growing, and numerous mitoses may be observed in all the cells 

 in the region of the mesenteric canal." 



Now although the process of invagination produces the 

 double-layered condition of the embryo of Amphioxus, and at 

 the same time the cavity of the archenteron, yet it is only the 

 anterior part of the archenteron that is formed in this way. 



There is a posterior point of the archenteron which is 

 formed, not by invagination, but by growth of the blastoporic 

 lips. This must be so, whether we accept Hatschek's or 

 Wilson's description of the secondary growing point. 



The exact line of demarcation between the two parts I have 

 no means of showing. It is not easy to say at what moment 



