235 
Cynthia 1 will prove very suitable for our purpose. There is no 
yolk in the young egg of Cynthia— the ovarian egg — and 
the neucleus is situated in the centre. The deposition of yolk 
takes place round another body to be found in the cytoplasm (pro- 
bably the attraction sphere). A peripheral layer of cytoplasm re- 
mains free from the yolk, but pigment granules of a yellow colour 
are deposited in this region. During the maturation changes, nuclear 
sap flows upward and forms a cap of cytoplasm at one pole of the 
egg, in which the chromosomes may be seen lying. Whilst matura- 
tion divisions proceed, the clear nuclear cytoplasm and the peripheral 
cytoplasm with yellow pigment both flow down to the opposite pole 
of the egg. The result is that the slate-coloured yolk is now at the 
upper pole whilst clear cytoplasm with more internally situated 
yellow pigment is to be found collected at the lower. Further 
changes take place in the distribution of these different substances 
as fertilisation takes place. It will be seen, therefore, at the outset, 
that the structure of the egg is not homogeneous and that different 
substances are actually visible. 
The first cleavage divides the egg into two equal cells. The 
second cleavage results in four cells, but the yolk is separated so that 
it all passes into two cells only. The third cleavage gives eight cells 
and the coloured substances are still further segregated. Two cells 
now consist almost entirely of grey yolk, two cells almost entirely 
of yellow pigment, and four cells contain almost only clear sub- 
stance. For our present purpose it is not necessary to follow the re- 
maining divisions. 
Now some authors regard the sequence of events in this 
development as indicating that the coloured substances in 
the cytoplasm are definite organ-forming substances which cause and 
control chemically the phenomena of development. It was found 
by Conklin that if one of the first two blastomeres was killed, the 
other one segmented as if its sister were still present, and hence only 
half a larva resulted. If three blastomeres were killed in the four- 
cell stage, the survivor, whichever it might be, gave rise only to an 
imperfect larva. In fact, what developed out of the surviving 
blastomere corresponded exactly to what would have developed had 
the three sister blastomeres remained alive. It appears demonstrated, 
therefore, in this case, that the organisation present in the egg - 
whatever it may be — cannot be divided into equal parts which are 
totipotent. The factors of development seem, at first sight, to be 
different from those of the sea-urchin’s egg. 
If development in this way were universal it might appear quite 
easy to demonstrate the probability of a three-dimensional machine. 
It is not, however, necessary, to my mind (even if physico-chemical 
factors are regarded as sufficient) to prove the existence of a three- 
1 Conklin. Orientation and Cell-lineage of the Ascidian Egg. 
Philadelphia, Series 2, vol 13, 19u5. 
Journ. Acad. Sc. 
