1889.] Segmentation of the Ovum. 473 
lachians, reptiles and birds, the yolk may not divide into cells 
as fast as the nuclei multiply, so that it seems that 
the presence of the deutoplasm, though it does not affect 
the nuclear divisions markedly, certainly impedes very much 
the division of the protoplasm, and consequently in these ova 
we find at certain stages of development a multinucleate yolk. 
The impediment is not encountered by the protoplasm of the 
animal pole, hence we see the animal pole segmenting while 
the yolk does not ; in this case the segmentation appears con- 
fined to one portion of the ovum, and accordingly such ova are 
termed meroblastic in contradiction to the holoblastic ova, m 
which the first cleavage furrows divide the whole ovum ; but 
the difference, it must be expressly remembered, is one of 
degree not of kind. 
The best known example of a vertebrate meroblastic ovum 
is, undoubtedly, the hen's egg. The so-called yolk or "yel- 
low " is the ovum ; the white and the shell are both adventi- 
tious envelopes added by the oviduct as the ovum passes down 
after leaving the ovary. The segmentation begins while the 
ovum is passing down through the lower part of the oviduct, 
and shortly before the formation of the shell commences. If 
an ovum from the upper part of the oviduct be examined, it is 
found to be surrounded with more or less white (albumen). 
Its animal pole is represented by a whitish disk from 2.5—3.5 
mm. in diameter and 0.30—0.35 mm. in thickness ; this disc is 
known by many names — formative yolk, germinal disc, cica- 
tricula, (Narbe, Hahnentritt, Keimscheibe, stratum s. discus 
proligerus). The animal pole consists chiefly of protoplasm 
and is peculiar only in its small size compared with the whole 
ovum ; it contains, when the ovum leaves the ovary, the egg- 
cell nucleus ; the ovum then matures; impregnation occursand 
finally segmentation begins. Viewing the ovum from above, 
we see the first furrow appear as a groove running across the 
germinal disc, though not for its whole width, and dividing it 
into halves; this furrow is developed in accompamment with 
the division of the segmentation nucleus. The primary furrow 
is succeeded by a second furrow nearly at right angles to the 
