No. I.] 



GERM-LAYERS IN CLEPSINE. 



109 



Semi-diagrammatic surface views of tht egg of Clefsine in different stages 

 of Cleavage. 



Diag. I. — The eight-cell stage, showing the relation of the embryonic axis 

 to the first two cleavage-planes. The arrow, 2-2, shows the median plane of the 

 embryo, and the four small arrows indicate the direction in which the four 

 micromeres have rotated on the axis of the egg. 



Diag. 2. — The posterior macromere (Diag. 1, x) has just divided into the 

 neuro-nephroblast x\ and the two mesoblasts x and xy. 



Diag. 3. — Same stage seen from the lower pole. 



Diag. 4. — Stage in which all the teloblasts and the blastodisc have been 

 formed. 



I-I and i-i. — First cleavage-plane; II-II and 2-2, second cleavage plane; 

 a, left macromere; c, median and anterior macromere; b, right macromere; 

 *, posterior macromere; ca, cc, bb, xx, micromeres derived from a, c, b, and 

 «', neuro-nephroblast; xy, left mesoblast; x (Diag. 3.), right mesoblast; nb, 

 neuroblast; nph, nephroblast; /, lateral (median in this stage) teloblast. 



