716 THE GENERATIVE ORGANS. 
been deposited, and which therefore, on any theory, contain 
germ-yelks—whether the spermatic capsules of the female con- 
tain sperm-cells or not. In such eggs, Henking sees a definite 
nucleus surrounded by a protoplasmic area, in which he figures 
nuclear spindles, and from which he says directive corpuscles 
are produced. It appears to be very generally admitted by 
those who have examined the phenomena, that the directive 
corpuscles are formed very soon after the eggs are deposited, 
and after the entrance of the sperm-cells into the yelk. With- 
out in any way wishing to offer an opinion as to the validity of 
the discoveries claimed in this direction by recent observers, I 
would remark that they are equally consistent with my own 
views and those of the writers themselves. It is possible and 
probable, I think, that the germ-yelk undergoes changes simi- 
lar to those described as soon as it enters the yelk, and that 
the simultaneous formation of a male and female pronucleus 
results ; these subsequently unite to form the first segmenta- 
tion nucleus. 
Some of the figures representing the phenomena observed, 
are certainly suggestive of a series of changes corresponding 
with those repeatedly observed in ova which exhibit more 
favourable conditions for observation. Others are, however, 
far from satisfactory, and scarcely appear to support the great 
superstructure of hypotheses which has been raised upon so 
slender a basis. 
Free Nuclear Formation.—Schwann held that cells originate 
by the differentiation of a structureless substance, ‘ blastema, 
which, according to its chemical qualities and grade of vitality, 
possesses a greater or less capacity of effecting the develop- 
ment of cells.’ But the experience of embryologists was soon 
found to be in opposition to his views. Prevost and Dumas 
observed the process of yelk segmentation in the frog in 1846, 
which led to the statement that the segments into which the 
yelk splits are cells. It was not, however, until 1855 that Vir- 
chow made his well-grounded statement, Ommis cellula e cellula, 
that every cell is developed from a pre-existent cell, which is 
the basis of the present almost universally accepted view. 
