MUSEUM OF COMPARATIVE ZOOLOGY. 111 
at the centre of the yolk in the first condition of the egg. It there be- 
comes more dense and divides. Of the two masses of unequal size, 
“ Toujours,” he says, “une des deux présente des proportions considé- 
rables par rapport 4 l’autre.” In fig. 3 Pl. XI., to which he refers for 
this phenomenon, this difference in size of the two is not well shown, 
and the condensed central region is not separated from the superficial by 
as strong a line of demarkation asin Ophiopholis. It is supposed that 
the condensed central part which is referred to in his description is the 
same as the slightly opaque or greenish centre of the Ophiopholis egg, 
but such an interpretation is open to doubt. The transparent superficial 
layer is not homogeneous throughout. At one pole on the outer surface 
of the transparent layer of the yolk, still fastened to it or not sepa- 
rated from the yolk cells, a single globule was observed, PI. I. fig. 2, d, 
possibly in the process of forming. This globule forms a slight elevation 
on the surface of the transparent layer, and a corresponding conical ele- 
vation was observed under it on the denser part of the vitellus.* Later 
in time a globule separated from the yolk was observed, and in later 
stages of development an additional globule is formed, Pl. II. fig. 3. 
The largest number of free polar globules observed was two. Polar glob- 
ules are not figured or mentioned in Ophiothrix by Apostolides. He 
speaks of them, however, in Amphiura. The superficial layer of the 
yolk appears to surround both cells, and in the contiguous surfaces of 
the two cells this layer is undivided, corresponding in its position with 
the plane of the first cleavage, Pl. I. fig. 3. This coincidence causes the 
two cells of the 2-cell stage to appear separated by a transparent layer, 
which at the same time unites them,t Fig. 3, 1 cl. pl. 
The formation of the 2-cell stage does not occur immediately after the 
sperm is added to the glass containing the ova. As in Echinarachnius a 
considerable time elapses after the mixture of the two elements before 
the formation of a 2-cell stage. The indications are that the first changes 
go on more rapidly in Ophiopholis than in Ophiothrix, as observed by 
* This conical elevation may be connected with the “ Dotterhiigel” of Fol and 
others. It has, however, no existence on the outer surface of the plasmic or corti- 
cal layer. A more acute histological examination of the single globule on the sur- 
face of the latter above the conical elevation is necessary before it can be stated 
whether it is a polar globule or a spermatozodn. I regard it as the first polar 
globule. 
t A characteristic connection of the two blastomeres is mentioned in the egg of 
Ophiophragma by Professor Nachtrieb. Whether it has a likeness near or remote 
to this condition in Ophiopholis, it is impossible for me to say. From his short 
description I am unable to compare the two genera in this regard. 
