244 A Sketch of Comparative Embryology. [ April, 
single body, to which the very appropriate name of segmentation 
nucleus has been given. Possibly a contraction of the impreg- 
nated egg always occurs, either during 
or immediately after the entrance of the 
spermatozoon, so that a space intervenes 
between the yolk and the envelops of 
the egg. As little heed has been paid 
to this point, we are uncertain about it. 
We thus have seen that a male and 
female element unite and make a single 
Hig: Eer of Nephelis perfect cell. This fact offers a very 
ne hours after waei m, strong support to the theory that cells 
male,—f, female ucleus ; 
p.g., polar globules ; pi Hert. Contain two sexual constituents in a 
wig. atent condition, and are therefore to be 
considered either hermaphroditic or sexless. 
It has long been known that the egg of every animal must be 
impregnated by the spermatozoa of its own species, while on the 
other hand reproduction is dependent upon a certain dissimilarity, 
the existence of which is well established, although its nature 
cannot be even hypothetically explained. When the parents have 
been closely related for several generations, the sexual products 
alter in such a way that they cannot produce a complete animal 
by their fusion, although fusion with an element from another less 
closely related individual is still effectual. From these circum- 
stances arises the necessity of cross breeding, a fact which has 
been so much discussed in public that I need not occupy further 
space to describe it. In a very few cases the genoblasts of nearly 
related species may unite efficiently, producing an animal par- 
taking of the character of both parents—in short, a hybrid. Such 
exceptions are, however, extremely rare. 
Since the offspring inherit the peculiarities of the parents, it is 
evident that the transmission must take place through the geno- 
blasts, and various theories have been propounded to account for 
it, but no view has yet been brought forward which can justly be 
termed satisfactory, not even excepting the theory of pangenesis. 
The formation of the impregnated egg, with its segmentation 
nucleus, marks the beginning of new cycle of life, for the cell SO. 2 
formed is endowed with a mysterious and remarkable power, 
which entirely distinguishes it from almost every other kind Of; 
cell known at present. The fertilized ovum is charged with free a 
