ON THE DIPLOOHOKDA. 389 



the hypoblast cells contain far more yolk, which stains well 

 with eosiu, the cell walls are much fainter, and are in some 

 cases indistinguishable (due to method of preservation), 

 whilst the nuclei are slightly larger and less regularly 

 arranged. 



In fig. 25 the blastocoele cavity is still seen, and it is usually 

 quite destitute of any structural contents. Occasionally I 

 have noticed a few fine strands of cytoplasm crossing it, or 

 even gathered into small masses, but in no case have I seen a 

 nucleus, or any body which stained with the dark blue tint of 

 hsemalum present in the blastocoele. 



Fig. 26 shows a median sagittal section of a stage a very 

 little later than fig. 12. Here the invagination is completed, 

 and the blastopore even in its length has considerably con- 

 tracted. The epiblast cells are slightly longer and more 

 abundant at the anterior end than elsewhere. The blasto- 

 coele cavity is still distinguishable and empty as before. Up 

 to this stage there is nothing which could be construed into 

 a trace of mesodermic elements in any part of the embryo. 



Fig. 27 is a median sagittal section of the stage represented 

 by fig. 13 or slightly earlier. Here we may note that the 

 hypoblast is in close contact with the epiblast in every direc- 

 tion. Whether due in any degree to a post-mortem contrac- 

 tion or not, the blastocoele cavity has disappeared altogether. 

 The blastopore has further narrowed in extent, and the 

 epiblast at its lips has commenced to grow inwards, following 

 in the wake of the invaginated hypoblast. 



Figs. 29 and 30 are median sagittal sections of figs. 14 and 

 15 respectively. In them the further formation of the 

 stomodifium is shown, fig. 30 indicating that the whole 

 " oesophagus " of the larva is formed in this way fi-om 

 epiblast. The gradual ventral growth of the pre-oral hood 

 is also clearly shown. One feature to be noted is the entire 

 absence of any indication in section of the ventral groove. 

 In transverse sections it appears merely as a superficial 

 depression of the cells, and it would appear that concrescence 

 of the respective hypoblast and epiblast of the two lips 



