Development of Holothiiria Floridana Pourtales 213 



In my preliminary communication (1907), I had not completed 

 this study by reconstruction from serial sections and thus a few 

 discrepancies were then published. I have called all appendages 

 developed, which clearly project from the skin and seem to be func- 

 tional, even if much contracted and very short. Those still buried 

 in the body-wall, often merely initial evaginations from the radial 

 canal, and then determinable only in sections, I have called huds. 



II. K^OTES o:^ THE General, Embryology. 



The formation of the polar bodies and fertilization are followed 

 by a total and approximately equal cleavage. The stage with four 

 blastomeres is reached at three hours, with sixteen blastomeres at 

 four hours and the blastula by the fourteenth hour. Then the forma- 

 tion of mesenchyme begins by cell-proliferation at the vegetative 

 pole, while at the same time gastrulation takes place. By the twenty- 

 second hour a plug of cells has gro^\ai out toward the blastopore, 

 from the blind end of the archenteron, dividing this sac into two 

 diverging diverticula of which the ventral constitutes the enteron, 

 and the dorsal, the vaso-peritoneal vesicle. In the second day the 

 vaso-peritoneal vesicle grows larger and begins to show the division 

 into hydrocele and enteroceie (PI. I, Fig. 2). During this day a 

 crescentic depression on the ventral surface marks out the initia- 

 tion of the peristome (PI. I, Fig. 1). This depression gradually 

 deepens and straightens, growing out to either side until it extends 

 entirely across the ventral surface of the embryo (PL I, Fig. 3). 

 The plane of the peristomial groove is at an angle of fifty degrees 

 with the sagittal plane of the adult holothurid. 



At this time spots of green pigment appear in groups, but later 

 are evenly distributed over the whole surface. Thus the broAvn 

 embryo gradually becomes greenish in color. During the second 

 and third days the ectoderm is ciliated and the embryo revolves 

 within the vitelline membrane. 



At the time of hatching the mouth has become established medianly 

 in the peristome, while the enteric canal has the characteristic dorsal, 

 left, and right loops (PI. II-III, Figs. 7-14). As the young holo- 

 thurid creeps about, it begins to eat the protoplasmic fragments in 



