AMERICAN NATURALIST. 
SEGMENTATION OF THE OVUM, WITH ESPECIAL 
REFERENCE TO THE MAMMALIA. 
"THERE follows after impregnation a short pause, and then 
the ovum begins its process of repeated division, which is 
known as the '' segtnefitation of the ovum;' the term having 
been introduced before it was known that each " segment " is 
a cell. The division or cleavage (Furchung) of ova was de- 
scribed by Prevost and Dumas, 1824,^ and again by Rusconi in 
1836. By usage, the term segmentation is restricted to the 
production of cells up to the period of development, when the 
two primitive germ-layers are clearly differentiated and the 
first trace of organs is beginning to appear. 
Segmentation nucleus. The impregnated ovum has a single 
nucleus which is known as the segmentation nucleus, and 
which is formed by the union of the male and female pronu- 
clei.' It is the parent of all the nuclei subsequently found in 
the organism, and participates actively in the process of seg- 
mentation. It is very much smaller than the nucleus of the ^g^- 
' Ed van Beneden in his first paper, Ascaris, 11, affirmed that there was no real 
union of the pronuclei in the impregnated ova of that species, but Carnoy, 18, 
showed that van Beneden's observations were incomplete, and Zacharias has stated, 
OO, that they are so defective as to be fundamentally erroneous in regard to im- 
portant phases, and he points out that in reality the eggs of Ascaris offer another 
proof of the actual union of the pronuclei. The impregnation in this Nematod has 
since formed the subject of numerous articles, see van Beneden and Neyt, 12, 
Carnoy, 182, Boveri, 15, etc., etc. 
