316 Thos. H. Montgomery. 



tion and enlargement of extraembryonic blastodermic cells in a 

 region where there is no mesoblast. Fig. 67c shows the details of 

 this process on higher magnification, the border of the embryonic 

 body being at the jDoint marked G. B; while Eig. 68g shows another 

 group of them in statu nascendi just lateral from the head lobe. 

 Their formation is very characteristic and not to be confounded with 

 that of any other cells : at certain points the extraembryonic cells 

 multiply, the nucleus of each enlarges and the cytoplasm still more 

 rapidly, then from the large and dense nucleus strings of chromatin 

 substance pass into the cell body until the latter contains a heavy 

 chromidial net (Figs. 67c, 68g). Thus, the cells come to assume the 

 appearance of those blood cells later found within the heart cavity. 

 These cells are unquestionably ectoblastic, for they arise in regions 

 where there is no mesoblast, and are at the start on the surface of the 

 blastoderm. On the other hand, there are no indications whatsoever 

 of blood cell formation from the mesoblast at this or later stages. 'As 

 they enlarge they sink below the blastoderm, and the stage following 

 this one will show how they move into the embryonic body by migra- 

 tion. The point of origin of the blood is, accordingly, that extraem- 

 bryonic area indicated in Figs. 63-66 by stippling. 



8. Stages Immediately Preceding Reversion. 



Here may be considered two slightly different stages, one of about 

 97 hours (Figs. 71-74, PI. VI), the other of about 108 hours (Figs. 

 78, 79, PL VII). 



The rostral appendages have fused to compose the rostrum {Ros., 

 Figs. 71, 72, PI. VI; 79, PI. VII), which is broad with its free end 

 directed anteriorly. Fig. 80b {Ros. ) shows it on median section and 

 Fig. 75b cut a little to one side of the midline, these figures elucidate 

 also the extent of its rostral mesoblast {R. Mes.) in antero-posterior 

 direction; and Figs. 76a (anterior to the rostrum) and 76b (in the 

 plane of the stomodaeum, Sto.) in transverse direction. The rostral 

 mesoblast sac (R. Mes., Fig. 80b) is much more extensive than the 

 cheliceral {Chel. Mes.). The unpaired rostrum has been produced 

 by the fusion of the paired rostral tubercles of earlier stages. 



The stomodaeum (Sto., Figs. 71, 72, PI. VT ; 79, PL VII) has 



