SEA MUSSEL MYTIEUS EDULIS. 



193 



a. Optical section of embryo 4 hours old. First polar body has divided making a total of three polar cells which still adhere 

 to embryo. Macromeres shown inclosed within the blastula (A/a in fig. 182/) have multiplied into smaller cells which more or 

 less fill the segmentation cavity. Cilia, too fine to be shown in the drawing, are developed and embryo begins to rotate slowly. 



b. Optical section of embryo 20 hours old, showingf ormation of gastrula and origin of germ layers. Epibolic process of gas- 

 tralation shown in fig. 182, h and /, now becomes typical embolic process which results in formation of entoderm, Ent, and a 

 blastophore, BL Erf is ectoderm , the mesoderm cells, Msd, which are scattered throughout the segmentation cavity, are derived 

 from the macromeres originally inclosed by epibolic process. Ciliaare well developed and trochophore larva swims about actively. 



c. Trochophorelarva 24 hoursold, showing flagcllum, Fl. 



d. Trochopore larva 42 hours old. showing formation of digestive tract and apical plate. An, anus or proctadeum. Mth, 

 mouth or stomadeum; St, stomach; VI, apical plate, anlarge of velum. 



e. Trochophore larva 45 hours old, showing shell gland, SG. 



f. Trochophorelarva 48 hoursold, showing first formation of shell, 

 p. Another larva 48 hours old, showing further growth of shell. 



h. Trochophorelarva 69 hoursold, showing straight hinge shell, Sh; anlageof velum, VI; and digestive tract. 

 i. Trochophorelarva 5 days old with well-developed velum, VI, and retractor muscle fibres, RV;Fl, flagellum; St, stomach. 

 Actual size of lavra 91.4 by 68.5 microns. 



