116 BULLETIN OF THE 
phiura by Metschnikoff (op. cit. Pl. III. fig. 6),* and that the two struc- 
tures v., supposed by him to be water tubes, correspond in position to 
the clusters of cells on each side of the invagination. These clusters in 
Ophiopholis were quite dense, and the vesicles, if they existed here, 
would be difficult to see. Metschnikoff says that in Amphiura these 
bodies are also difficult to see through the “Cutiszellen ” (mesoderm 
cells), and that later in normal development one is lost. He was able to 
observe that one of these bodies in Amphiura develops into the water 
tubes of the adult. It is not wholly certain that similar bodies do not 
exist in Ophiopholis, Pl. I. fig. 14, @ cl., where clusters of amceboid cells 
make observation on live material somewhat difficult at these points. 
The bilateral arrangement of the budding cells in the cavity of the 
blastosphere and the shape of the larva give to it a marked bilateral sym- 
metry even at this early stage. The pole of invagination may be called 
an anterior pole, while the cells on each side indicate the sides of the 
larva. One hemisphere of the gastrula is flattened; the opposite is 
more rounded. The former may be called the ventral, the latter the 
dorsal surface. 
At seven o’clock on the day following the spawning the invagination, 
which forms the archenteron, has extended about half way down the 
cavity of the blastosphere, Pl. I. fig. 15. Almost the whole of the 
second twenty-four hours is occupied by the changes which accompany 
the infolding of the archenteron.T 
The pole of the infolding slowly sinks into the cavity, carrying with it 
at this point the shell of cells, or that part of the blastoderm which is to 
form the wall of a digestive canal. The larva has become very much 
flattened on the ventral side, so that when seen from the pole of invagina- 
tion the lateral diameter is twice that at right angles to it in the same 
plane. As we have arbitrarily called the longest diameter, when seen 
from the pole of the blastopore, the lateral, a name which seems appro- 
priate, not only on account of the bilateral symmetry which the larva at 
this early age has, but also from the fact that from its extremities form 
the two calcareous rods and fleshy arms, known as the lateral arms, we 
may speak of the other diameter as the dorso-ventral. The dorso-ventral 
diameter connects the dorsal and ventral side of the larva, which are 
* Fig. 6 is a little older. The mode of origin of these vesicles was not observed 
by Metschnikoff. Their position relatively to the mouth of the larva is somewhat 
exceptional. 
+ The time occupied to form the gastrula of Ophiophragmais about the same as 
in Ophiopholis. Cf. Nachtrieb, op. cit. 
