Fig. 5

Sections of P. californica embryos during inversion and subsequent development. a-t In situ light micrographs of toluidine blue stained semi-thin sections of fixed embryos inside their mother spheroids. a-h Mid-sagittal sections of successive stages of inversion. The start of inversion is at time zero (t = 0 min). a t = −3 to −5 min before the beginning of inversion. b t = −0.5 to −1 min. c t = 3 to 4 min after the beginning of inversion. d t = 6 to 6.5 min; asterisk: bent peripheral cell. e t = 7 to 8 min. f t = 10 to 11 min. g t = 12 to 13 min. h t = 14 to 16 min; end of inversion. i-l Transverse sections of stages after completion of inversion. i t = 20 to 25 min after completion of inversion. j t = 25 to 30 min after completion of inversion. k t = 40 to 50 min after completion of inversion. l Juvenile shortly after release from its mother spheroid. m-p Sections at right angles (transverse) to the image planes shown in (a, h, i); the positions of the image planes for (m-p) are indicated in (a, h, i), respectively (dashed yellow lines). m t = −3 to -5 min. n t = 14 to 16 min. o t = 14 to 16 min. p t = 30 to 40 min. q-t Magnified images of the framed areas in (a, c, I, n), respectively. q Teardrop-shaped cells. r Spindle-shaped cells. s Elongated teardrop-shaped cells. t Truncated pyramid-shaped cells. Scale bars: (a-p) 20 μm; (q-t) 10 μm