194 HIROSHI OHSHIMA 
is, as observed by Edwards (12, p. 213), reached at the 
fourteenth hour. 
Before invagination begins mesenchyme cells are formed by 
the active proliferation of the cells at the vegetative pole 
(Pl. 8, fig. 5). Having become free from the wall, these cells 
wander into the blastocoele, some lying attached to the wall near 
the animal pole (Pl. 8, fig. 6, me). 
The mesenchyme-formation begins, according to the species, 
either before or after the invagination, or sometimes at the same 
time as the latter. In C. frondosa and C. echinata 
the mesenchyme-formation precedes the mvagination. The 
same is true for C. planci in normal cases (Selenka, 45) 
but it may occur afterwards (Ludwig, 22). In H. tubu- 
losa and H. floridana both the processes occur at the same 
time, while in Synaptids the mesenchyme cells are formed from 
the tip of the already formed archenteron. Ludwig (21, 
1889-92, p. 258) noticed this fact and concluded that these 
differences are proportional to the rapidity of development. 
Thus in a form whose development is rapid mesenchyme is 
formed later, and vice versa. I may point out further that in 
those forms where the mesenchyme-formation takes place 
early the cells are generally very numerous and they readily 
fill up the blastocoele, while in those where invagination precedes 
the mesenchyme-formation the cells are generally few. 
As to the origin of the mesenchyme Ludwig (21, p. 258) 
surmised that some mesenchyme cells may arise from the 
blastoderm in other places than that where the future endoderm 
is situated, and from his study in C. planci (22, p. 605) he 
claimed to have proved this statement. His view could not 
be confirmed by Newth (86, p. 635), while Clark (7%, p. 61), 
in his observations on Synaptula hydriformis, felt 
‘no hesitation in affirming that the mesenchyme arises 
exclusively from the endodermal cells’. It is highly 
probable that Ludwig saw those cells attached to the 
future ectoderm, as I have mentioned above. I could not, 
however, find any positive evidence to support his view, and 
in contrast to the vegetative part where many mitotic figures 
