xlvi 
THE YOYAGE OF H.M.S. CHALLENGER. 
may be seen in them a conical projection attached by its base to a point on the inner 
surface of the wall, whence it extends deep into the interior. 
These bodies are certainly not cells ; they take no direct participation in the 
development of the embryo, and Kleinenberg, who has traced them from small globular 
condensations of the plasma, assigns to them the name of “ pseudo-cells.” They seem 
to have the significance of reserve matter, and gradually disappear. 
The egg still continues to increase in size, but the outrunning extensions of its 
plasma are gradually withdrawn, and the egg loses its amoeboid form, and becomes 
rounded into a broad ovoid. 
About the time when the formation of pseudo-cells has ceased the germinal vesicle 
and germinal spot begin to undergo a disintegration, and finally disappear long before 
the commencement of fecundation. The remaining cells of the ovary also become 
disintegrated and are manifestly used up as nutriment for the ovum. 
By the pressure of the subjacent egg the ectoderm which lies over it becomes 
protruded in the form of a sac, in the summit of which a narrow orifice makes its 
appearance. Through this the egg is forced out into the surrounding water, though 
it still continues for some time to hang to the parent Hydra by a small part of its 
surface. It is now that the naked egg is exposed to the influence of the spermatozoa 
diffused through the water, and. becomes fecundated. 
The segmentation which now begins in the ovum is in accordance with the 
common law of binary cleavage, except that when the cleavage advances to a certain 
stage it loses somewhat of its regularity, some of the segmentation spheres cleaving 
less rapidly than the others. During this process pseudopodia are developed from the 
surfaces of cleavage, but these at a later period disappear. When the number of the 
segmentation spheres has reached thirty -two the surface of the egg has the mammillated 
condition of the mulberry form. Later on this disappears and the egg becomes quite 
smooth. 
When the segmentation has been completed two kinds of cells may be distinguished 
in the germ. One of these consists of elongated prismatic cells resembling those of 
a cylinder epithelium. They form a one -layered stratum on the surface of the germ. 
The other consists of shorter cells rendered polygonal by mutual pressure. These form 
the main mass of the germ. All these cells are naked plasma masses, at first destitute 
of nucleus, but at the end of some hours nuclei make their appearance in the superficial 
cells, and soon after in the deeper ones. It seems certain that these nuclei arise inde- 
pendently in cells previously destitute of them. 
Soon after these occurrences it is seen that the albuminous corpuscles, chlorophyll 
grains, and pseudo-cells contained in the superficial prismatic cells have retreated into the 
deeper parts of these cells, while the nucleus remains for some time immediately beneath 
their free surface, but ultimately disappears. 
