J. W. Jenkinson 
203 
The phenomenon is therefore of fairly widespread occurrence, though it must 
be remembered that there are cases in which the bilateral symmetry of the egg is 
fixed, apparently unalterably, in the unfertilized egg, for instance, in Cephalopods 
and many Insects, and secondly, that in parthenogenesis, the determination of 
symmetry, if not pre-existent, must be due to other causes. 
But however the definite arrangement of cytoplasmic substances, on which the 
bilateral symmetry of the egg depends, may be brought about, modern experimental 
research has made it increasingly clear that it is upon these substances that at any 
rate the first differentiation of the embryo depends. Thus, to select only one or 
two examples, the Plane of Symmetry in the Frog becomes the Sagittal Plane of 
the embryo, in the absence of external interferences, the animal pole being 
approximately at the anterior end, the grey crescent on the dorsal side. 
In Dentalium the polar lobe contains the material for the apical organ and 
trunk region of the larva ; removal of the lobe entails the absence of both these 
structures. 
Derangement of the micromeres brings about a multiplication of the sense 
organ of the Ctenophor embryo (Fischel) ; an isolated vegetative blastomere of a 
Nemertine will give rise to a larva with an archenteron but without an apical 
organ, an animal blastomere to one with an apical organ but no archenteron 
(Zeleuy), and in Sea-urchins vegetative blastomeres gastrulate more readily than 
do animal cells (Driesch). 
By the removal of one of the two posterior cells — containing the yellow stuff — 
of Cynthia, an embryo is produced which is complete anteriorly, with nerve plate, 
notochord and anterior mesenchyme, but possesses posterior mesenchyme and 
muscles upon one side only. 
The substance which in these cases is necessarily associated with the formation 
of some particular organ, need not be, however, originally present in what will 
become its ultimate situation. Thus the stuff for the apical organ of Dentalium is 
at first situated in the polar lobe, subsequently migrating, between the first and 
second divisions, into the animal hemisphere. Though the organ-forming sub- 
stances may be said to be in a sense preformed, they are not therefore necessarily 
prelocalized. 
Speaking generally, a very definite relation may be noticed between the 
symmetry of the egg and that of the embryo, even where experiment has not, as 
yet, shown the necessary connection between this or that substance, and this or 
that organ. In a very large number of cases the animal pole is anterior, while the 
blastopore closes at the vegetative pole (Turbellarians, Annelids, Mollusca other 
than Cephalopods). In Cephalopods the animal pole is dorsal, the more convex 
side of the bilateral egg anterior, and a similar relation obtains in many Insect ova. 
In Cynthia the animal pole is ventral, the yellow side posterior. In Ascaris megalo- 
cephala, the plane in which the first four cells all lie is sagittal, while the vegetative 
germ-cell slips round to what will be the posterior side. The relation between 
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