DEVELOPMENT OF CUCUMARIA ECHINATA O17 
side so soon after the separation into two from the original 
single vesicle that he at first overlooked this separation. 
Ludwig (22, pp. 609, 611) observed in the fourth-day larva 
that the right and left enterocoeles extended around the gut 
so as to meet and break through on the ventral side, while they 
remain separate on the dorsal side. At the end of the sixth 
day the rudimentary mesentery begins to bend, the last section 
lymg on the right ventral side of the body. From Oster- 
gren’s comparative study in the Dendrochirotae (41, 1898) 
it has been shown that the last section of the mesentery does not 
lie on the right side of the mid-ventral radius, but with the 
exception of the Psolinae always on the left side. Ludwig 
was wrong in this respect. 
The above feature is essentially the same in the Synaptids. 
I may only point out that the pointed ends of the two entero- 
coeles unite on the right side of the mid-ventral line 
(Reimers, 48, p. 280). 
Blastocoele Jelly and Mesenchyme.—By the 
time that the dipleurula is reached the pre-oral hood is filled up 
with blastocoele jelly. It consists of a structureless gelatinous 
substance and a few sparsely arranged mesenchyme cells sus- 
pended in it. The former stains with plasma-dyes and often 
shows a netted appearance in some fixatives. This substance 
is seen most developed in the doliolaria stage, while near the end 
of the late doliolaria it gradually diminishes, probably being 
absorbed as nourishment. 
Most of the other mesenchyme cells gather thickly around the 
hydrocoele, enterocoele, and gut, without, however, forming a 
definite cell-layer of any kind. Others lying below the ectoderm 
form a loose connective tissue of cutis. Metschnikoff 
(29, p. 4) showed in a Synaptid the origin of the cutis from 
mesenchyme. Selenka (45, p. 169) opposed this view, claim- 
ing that mesenchyme gives rise to musculature only. Later, he 
(46, p. 57) corrected his former view admitting that mesenchyme 
gives rise to connective tissues, and on the other hand that the 
musculature of some parts originates from other sources than 
the mesenchyme. 
