DEVELOPMENT OF HALISARCA LOBULARIS. 621 



Figs. 27 and 28. — Sections of the same embryo in the last stage of folding 

 observed. 



Fig. 29. — Constituent cells of the hypoblastic and epiblastic layers of the 

 wall of the embryo, shown in Fig. 28. Zeiss, E Obj., No. 4 oc. 



Fig. 30. — Cells of a similar embryo, more advanced. The nucleus has 

 retreated to the inner end of the cells, and a flagellum has been produced. 



Fig. 31. — One of the foregoing cells, more highly magnified. 



Fig. 32. — A third section of the same embryo as that from which Figs. 27 

 and 28 were obtained. 



Figs. 33 to 38.— A series of sections of a single embryo, disclosing irregu- 

 larities which would not have been conjectured had the section Fig. 33 alone 

 been observed. 



Fig. 39. — Section of another abnormal gastrula. 



