DEVELOPMENT OF CUCUMARIA ECHINATA 195 
are to be met with, no such thing is found in the ectoderm. 
Newth observed some enucleated cytoplasmic droplets 
attached to the ectoderm. All I have seen were nucleated 
cells showing no notable difference from other mesenchyme cells 
suspended in the jelly. However, I cannot deny the réle that 
the ectoderm plays in mesenchyme-formation in a later stage 
of the gastrula, followed by the appearance of stomodaeum, as 
will be described below. 
Selenka (45, pp. 160-1, 168) observed some peculiar cells 
consisting partly of those detached from the blastoderm and 
partly of those which arose from subdivisions of the former, 
and called them ‘ Mesodermkeim ’. Every subsequent observer, 
however, denies their presence. 
. 10. GastRULA. 
Invagination begins early in the morning of the next day, 
i.e. about at the fifteenth hour. The larva gradually increases 
in length in accordance with the growth of the mvaginated 
archenteron and the multiplication of mesenchyme cells. It 
swims with the apical end forwards, at the same time rotating 
around its longitudinal axis. Cilia usually beat towards the 
oral pole. According to Ludwig (22, p. 605) the gastrula of 
C. planci is complete at the end of the second day, while 
Des Arts (2, p. 8) records that in C. frondosa the gastrula 
is formed as late as on the tenth day. Newth (86, p. 688) 
noticed in C. saxicola that the direction of rotation mostly 
changes, and at the gastrulation is the reverse of that seen in the 
blastula. 
The invaginated pit is beset with especially long marked cilia 
(Pl. 8, figs. 6, 7, ¢), which remain forming a bundle attached to 
the end of the archenteron for some period, still being visible 
even when a slight twisting has occurred in the archenteron 
(Pl. 8, fig. 8, c). The cells of the archenteron increase very 
actively, which fact is shown by many mitotic figures lying 
always near the surface and parallel to it (Pl. 8, fig. 7). The 
top of the archenteron shows no definite cell boundaries 
on the side towards the blastocoele; it here assumes the 
NO, 258 P 
