ON THE CJA8T11ULAT10N IN TETROMYZON. 



Explanations of Plates. 



List uf Abbreviationy. 



a. h Animal hemisphere. 



arxn Archenteron . 



b.(/ Boundary groove. 



bj) Blastopore. 



hp.l Blastoporic lip. 



con Conical eminence. 



ch Chorion. 



em.s Embryonic shield. 



(J. m Gastral mesoderm. 



i. mic. c^j... Inner micromeric epithel- 

 ium. 



I. mac. ey>..Lateral irregular epithel- 

 ium. 



niacin Macromeres (macromeric 



layer). 



mac. cyA... Macromeric epithelium. 



micm Micromeres (micromeric 



layer). 



mic.ep. ...Micromeric epithelium. 



m.fj Medullary groove. 



p.m Peristomal mesoderm. 



s.c Segmentation cavity. 



v.li Vegetative hemisphere. 



vl.hp Ventral lip of blastopore. 



+ Boundary between micro- 

 meric and macromeric 

 epithelia in archenteric 

 roof. 



.J. Anterior blind end of 



archenteron. 

 ,.^ Disturbed part of macro- 

 meric cell-mass. 



% Lower limit of micromeres. 



^^^ Flattened part of vege- 

 tative hemisphere. 



