FATE OF BLASTOPORE, ETC., IX CHELOXIA. 4 J 



h is two sections behind g. Here we see the last vestiges of 

 the posterior wall of the medullary canal. Otherwise it 

 is very much like g. 



i is two sections behind /(. There is no longer any trace of 

 the posterior wall of the neurenteric canal. The groove, 

 etc., are very mach as in g and h. 



In the next section behind i, there is a sudden falling in 

 the heio'ht of the section. We must consider the " End- 

 wulst " as extending from c backwards to that section. 



j is three sections behind /. The groove is wider than in 

 front, and in it, there is a small projection from the 

 bottom — the anterior end of the yolk-plug. 



k is five sections behind j. The yolk-plug has gradually 

 been enlarging from j backwards and in this section we 

 see a distinct yolk-plug separated on each side by a groove 

 from the epiblast which dips down to fuse with the mass 

 below. This is substantially like Series I,/, and Series 

 II, /, and shows conclusively that compared with those 

 series, the yolk-plug has shifted its position to one some distance 

 behind the posterior end of the medidlarij canal, the interven- 

 ing space between its original and present position being 

 traversed in the median line by a groove on each side of ivhicli 

 the epiblast is proliferating cells to the subjacent mesoblast. 



1 is four sections behind k. The yolk-plug is lower and the 

 groove on each side of it is shallower. 



m is six sections further back. The epiblast now stretches 

 over the entire upper surface, but the backward continua- 

 tion of the yolk-plug is still marked by a slight elevation 

 (included between two stars in the figure) and is fused 

 with the mesoblast at one point. After this section, the 

 layers become independent. 



It is hardly necessary to point out that of the above sections, g, 

 /(, and '/, are very mnch like sections through the jn-imitive streak of tlie 

 vertebrates in general. In fact I take the region represented by these three 



