310 W. WOODLAND. 
for about a quarter of the distance to the equatorial line, and 
the researches of numerous other observers on this and other 
genera—Hchinus, Hchinocyamus, Toxopneustes, 
Ophiopholis, Echinarachnius, Arbacia, ete.—tend to 
confirm this conclusion. These budded-off mesenchyme cells, 
though free in the sense that they are not attached to adja- 
cent structures, are yet not free in the sense that they are at 
liberty to wander into all regions of the blastoccele ; on the 
contrary all observations show that they take up a very 
definite position in this cavity. In H. esculentus, e.g. as 
the accompanying semi-diagrammatic figures (text-figs. 3—6) 
TEext-FIG. 5. Text-Fie. 6. 
indicate, they first aggregate in two positions, situated one on 
each side of the larva, where they form two elongated strands 
lying close to the body-wall and running parallel to the long 
axis of the gastrula. Later these two longitudinal cellular 
strands, which start from the posterior end of the larva, be- 
come connected by the formation of two chains of cells (in 
single series), which, extending round the archenteron on 
both sides and in each case joining on to the bases, i. e. pos- 
terior extremities, of the strand, thus form a complete circle 
(blastoporic ring) of cells lying immediately underneath the 
gastrula wall where it bends in to form the archenteron 
(text-figs. 3, 4). 
From the fact that these mesenchyme cells thus take up 
their position at all those regions of the body-wall where 
