diameter, and thus are only half the size of the mammal ova, and have
no distinctive features. The clear protoplasm of the mature ovum is
made so turbid by the numbers of dark granules of food-yelk or
deutoplasm scattered in it that it is difficult to follow the process
of fecundation and the behaviour of the two nuclei during it (Chapter
1.7). The active elements of the male sperm, the cone-shaped
spermatozoa, are similar to those of most other animals (cf. Figure
1.20). Fecundation takes place when these lively ciliated cells of the
sperm approach the ovum, and seek to penetrate into the yelk-matter or
the cellular substance of the ovum with their head-part--the thicker
part of the cell that encloses the nucleus. Only one spermatozoon can
bore its way into the yelk at one pole of the ovum-axis; its head or
nucleus coalesces with the female nucleus, which remains after the
extrusion of the directive bodies from the germinal vesicle. Thus is
formed the "stem-nucleus," or the nucleus of the "stem-cell" (cytula,
Figure 1.2). This now undergoes total segmentation, dividing into two,
four, eight, sixteen, thirty-two cells, and so on. In this way we get
the spherical, mulberry-shaped body, which we call the morula.

The segmentation of the Amphioxus is not entirely regular, as was
supposed after the first observations of Kowalevsky (1866). It is not
completely equal, but a little unequal. As Hatschek afterwards found
(1879), the segmentation-cells only remain equal up to the
morula-stage, the spherical body of which consists of thirty-two
cells. Then, as always happens in unequal segmentation, the more
sluggish vegetal cells are outstripped in the cleavage. At the lower
or vegetal pole of the ovum a crown of eight large entodermic cells
remains for a long time unchanged, while the other cells divide, owing
to the formation of a series of horizontal circles, into an increasing
number of crowns of sixteen cells each. Afterwards the