STUDIES IN SPICULE FORMATION. 311 
sharp curvatures are formed—the two longitudinal strands, 
e. g. being situated in the two corners, one at each end of the 
flat side, of the gastrula when this is viewed in a transverse 
plane (text-figs. 5 and 6), and the ring of cells being placed, 
as above mentioned, just where the posterior wall bends in 
to form the archenteron, i.e. round the blastopore,—one who 
- had not previously observed how strictly localised the area: of 
immigration is, would with reason suppose that the cells had 
been budded off in these regions—the cells being squeezed in, 
so to speak, by the foldings of the wall. But this is certainly 
not the case, as all observers agree. At the same time, it is 
an undisputed fact that in E. esculentus, and most, if not 
all, other genera, all these free mesenchyme cells migrate into 
all the corners of the gastrula, thus giving rise to the con- 
formation described above. As Théel (21), who has observed 
the process in living larve, says: “It is a sight of the 
greatest interest to follow these cells, to see how they move 
towards these two places [the positions of the two longi- 
tudinal strands] as by word of command.” 
The cause of this directed migration is not easy to 
ascertain. Herbst attributes it to the desire for oxygen on 
the part of the cells, and certainly, if we imagine oxygen as 
uniformly diffusing through the gastrula walls, those cells 
situated in corners will doubtless obtain the greatest share. 
Driesch (8) also advocates a chemotactic solution to the 
problem, and he performed an interesting experiment in 
connection with this subject. Stated briefly, this experiment 
consisted of shaking up Hchinoid gastrule in a test-tube in 
such a manner as to displace the mesenchyme cells forming 
the blastoporic ring and two longitudinal strands from this 
their normal disposition and to cause them to become 
irregularly dispersed throughout the blastoccele: the result 
was that, after the lapse of a certain time, Driesch found 
that the cells had returned to their original position in the 
blastoccele, and, as if nothing had happened, proceeded in 
their further development quite normally. 
One interesting fact concerning the cells of the blastoporic 
vou. 49, PART 2,—NEW SERIES. 22 
