126 BULLETIN OF THE 
The whole process of cleavage occupies about ten hours.* A rota- 
tion of the spheres of segmentation according to A. Agassiz occurs in 
Strongylocentrotus. This was not observed in Echinarachnius. Through- 
out all the changes the egg is enclosed in the capsule, cap, which has 
been mentioned in the unsegmented egy. 
Shortly after the end of the first half day after fecundation, the blas- 
tomeres arrange themselves superficially about the segmentation cavity, 
forming a hollow sphere, which is the blastosphere, Pl. II. fig. 14. Mi- 
nute cilia, which are long and fine, appear over its surface, and the egg 
begins to rotate and fret against the sides of the envelope or egg capsule, 
which closes it in. There is no solid morula stage, but a true blastula 
is immediately formed. At this time a thickening of the blastoderm at 
one pole takes place, the outline becomes more pyriform, Pl. II. fig. 15, 
and at the truncated pole a collection of pigment of deeper color than 
in the remainder of the ovum congregates. This increase in thickness 
of the cells at one pole is indicative of the formation of a gastrula 
mouth at that pole. Immediately after the thickening of the blasto- 
derm an infolding begins to take place at this pole, Pl. II. fig. 16. By 
this infolding, ga, the layer of cells which form the walls of the cavity, 
or the blastoderm, are infolded, and form the hypoblastic layer, or walls 
of a gastrula stomach. The infolding is at first very slight, but the 
increasing age of the embryo carries the walls deeper and deeper into 
the cavity. 
With the first indication of an ingrowth of the gastrula stomach, or 
archenteron, we find budding off into the segmentation cavity certain 
cells, a cl, which from their form, position, and other characters, are 
called the amceboid or mesoblastic cells. They give rise to important 
structures, which later appear in the embryo, between epiblast and 
hypoblast, and which belong to the middle layer or mesoblast. Prouho Tf 
finds in Dorocidaris that these cells are not all the same. When his 
paper came into my hands it was too late to verify in Echinarachnius 
what he finds in Dorocidaris. At the time my observations were made 
all the so-called amoeboid or mesoblastic cells were regarded as the same 
in character, and although I supposed that they did not all form the 
same structures, their differentiation in form was thought to take place 
much later than in the gastrula stage. These cells form on each side of 
an axis, passing through the gastrula mouth or primitive infolding. 
Their bilateral arrangement was not as marked as in Ophiopholis. They 
* Rate of growth in water of unrecorded temperature. 
+ Comp. Rendus, ci. pp. 586-388. 
