SEA MUSSEL MYTILUS EDUUS. 



191 



182 



Fig. 182. — Egg at various stages. X 375. 



a. Egg at stage 4 minutes later than 181/. Three nearly equidistant furrows have formed, separating formative poleinto 

 two masses and setting nutritive pole off as single mass. 



f». Two minutes later. Nutritive pole has enlarged to form macromere. Left mass at formative pole is about to be cut off 

 as first micromcre. 



c. One minute later, showing fusion taking place between macromere and yolk mass to right of formative pole. First micro- 

 mere is now completely separated by a furrow. 



d. Two minutes later, showing continuation of same process. 



e. Ten minutes later with macromere and microinere in resting stage. Their nuclei are visible as clear spots in centers of the 

 cells. 



/. Ten minutes later. Macromere has given off a second micromcre, and first micromere has divided into two as indicated 

 by arrows. 



p. Three minutes later. 



/;. Twelve minutes later. 



»'. Two minutes later. 



j. Fifteen minutes later, showing cap of micromeres gradually growing down over macromere. 



k. Optical view of a little later stage, showing segmentation cavity, SC. 



L Few minutes later. Macromere now completely surrounded by micromeres, Mi, and has divided into two equal cells. 

 Ma, that are apparently forerunners of the mesoderm. 



