«S89iJ Embryology. 507 
three planes, simultaneously and successively, with but little 
coherence, forming a delicate blastula, the cells of which are 
separated from each other by interspaces, and joined together 
by very slender protoplasmic bonds. Certain cells of this 
blastula-like organism grow directly into germs with exag- 
gerated dimensions. The wall of the Volvox blastula 'is prob- 
ably ectodermicand entodermic in its homologies, gastrulation 
is still to occur, but it is interesting to observe that al- 
ready the germs are produced in a little more than one hemi- 
sphere only, which probably corresponds to the ectodermic 
portion of a Coelenterate, while the empty, anterior, directive, 
and sensory pole is homologous with the entoderm of the lat- 
ter. The tendency of the germ-cells to originate from the 
ectoderm in some Coelenerates, therefore, may have an ances- 
tral significance. 
The over-growth of Protozoan or Protistan forms probably 
gave rise, through a series of segmentations, directly to such 
types as Volvox, and simulating the planula or blastula more 
or less closely. Gastrulation, under its various guises, as well 
as proliferation and delamination, also followed, with their 
consequences, which led to the direct development of the var- 
ious forms of ciliated larvae, at once ready to feed, undergo 
metamorphoses, and share in the struggle for existence. 
This first larval development was probably rapid, and due 
to the same causes as are still seen to be operative in the de- 
velopment of ova, namely, rapid segmentation. The accumu- 
lation in the egg of a mass of plasma in excess of the aver- 
age of its fellow cells or individuals,laid the foundation for the 
first and most primitive type of segmentation, namely, the 
holoblastic, before any yolk was added to the ovum, as is seen 
m the development of Volvox. This coherent aggregate 
was now an individual, ready to begin the struggle for exist- 
ence, and with infinite capacity for variation, and with an aug- 
mented power of reproduction. 
The ovum, according to this hypothesis, becomes the con- 
servative factor in biological evolution in a new sense, while 
the male element imparts the power to undergo rapid segmenta- 
tion, and to quickly achieve the larval state, when the interac- 
tion of the organism and the environment can be brought into 
play. The physiological activities of such plasmic aggregates 
as an oosperm are at first al'most wholly karyokinetic, and but 
slightly metabolic ; this renders possible the later and immedi- 
ately subsequent anabolism through which further growth and 
