FERTILISATION OF THE OVUM. 17 
After a period of quiescence, which succeeds the fusion of the male and female 
pronuclei, a period of activity supervenes, during which repeated divisions of the 
impregnated ovum result in the production of a solid mass of cells called a morula. 
The divisions are mitotic, and all the phenomena associated with mitosis are readily 
observable in properly prepared specimens. 
The planes which separate the several segments of the divided ovum in its 
various stages are termed the “planes of see- 
mentation, and in some animals, possibly in 
all, the first plane by which the ovum is divided 
into the first two daughter cells coincides with 
the line along which the pronuclei passed to 
their fusion; it indicates the future mid-axial 
or mesial plane of the body, the descendants of 
the cell lying to the right of it being developed 
into the right half of the body, and those of the 
cell to the left into the left half. There is no 
proof, however, that this occurs in mammals; 
all that is definitely known is that the first 
division separates the ovum into two parts of 
unequal size but of similar colour and structure. 
The second plane of segmentation is at right 
angles to the first and it separates the two 
daughter cells into four grand-daughter cells, of 
which, in some cases, two may be larger and two 
smaller. The subsequent divisions occur irregu- 
larly, and they result in the formation of 
numerous cells (blastomeres) which apparently 
only differ in size in the rabbit, but which also 
differ in appearance in many mammals. They 
are mixed together so irregularly that it is im- 
possible to distinguish the descendants of one 
daughter cell from those of the other, and in 
this, the morula stage, there is frequently no 
indication of any separation of the cells into 
layers. In the meantime the polar bodies have 
disappeared. 
The next phenomenon of importance is the. 
appearance of a cavity—the segmentation cavity 
—in the morula; the ovum assumes a vesicular 
character, and is now termed a blastula. Simul- 
taneously with the appearance of the cavity the 
cells of the morula are arranged in two groups Fic. 10.—Converston oF THE MoRULA To 
—an outer and an inner. The cells of the outer THE BLAsTULA. 
group form a layer, the primitive ectoderm or — Potmation of blastodermie vesicle and 
epiblast ; those of the inner group remain massed Aipeunaneecoe Dea cavity Cane 
together and constitute the inner cell mass. The ‘attachment of inner cell mass to ectoderm 
two groups are in contact at one pole of the pitas nee of Cae ; Boe sisuen a 
ovum, and it is in this region that the embryo ji Grbilaminar blastodermie vesicle, BO, 
develops. Though at first the inner cell mass is  Blastodermic cavity ; EC, Ectoderm; EE, 
not laminar, it soon becomes so by the migration Embryonic ectoderm ; EN, Entoderm ; I, 
and flattening of its cells; the inner layer thus ee sella ne, Bt grieiiiaelone ayia 
formed is termed the entoderm or hypoblast. 
The ectoderm and entoderm together constitute the blastoderm or blastodermic 
membrane, which is bilaminar, and the vesicle of which they form the wall is no 
longer spoken of as the blastula, but as the blastodermic vesicle. 
A 
In the rabbit the embryonic part of the ectoderm is formed from the outer cells of the inner 
cell mass which unite with the corresponding part of the primitive ectoderm (Rauber’s layer). 
This, however, is merely a peculiarity found in the rabbit and some other mammals; more 
generally the whole of the inner mass becomes entoderm. 
a 
