INTRODUCTION 9 
of the ectoderm over the optic cup the primordium of the lens, 
etc. Primordia are, therefore, of all grades, and each arises 
from a primordium of a higher grade of generality. 
The emergence of a primordium involves a limitation in two 
directions: (1) it is itself limited in a positive fashion by being 
restricted to a definite line of differentiation more special than 
the primordium from which it sprang, and (2) the latter is limited 
in a negative way by losing the capacity for producing another 
primordium of exactly the same sort. The advance of differen- 
tiation sets a limit in all cases, in the manners indicated, to sub- 
sequent differentiation, a principle that has been designated by 
Minot the law of genetic restriction. 
This law has not been sufficiently investigated in an experi- 
mental fashion to demonstrate its universal validity, but enough 
is known to establish its general applicability. A very impor- 
tant property of primordia in many animals is their capacity 
for subdivision, each part retaining the potencies of the whole. 
Thus, for instance, in some animals two or several embryos may 
be produced from parts of one ovum. Similarly two or more 
limbs may be produced in some forms by subdividing a limb- 
bud, ete. 
V. GENERAL CHARACTER OF GERM-CELLS 
As already remarked the ovum and spermatozo6n have the 
character of single cells in all animals. They are, however, 
specialized for the performance of their respective functions. 
The ovum is relatively large, inert, and usually rounded in form. 
Its size is due to the presence of a sufficient quantity of proto- 
plasm to serve as the primordium of an embryo, and of a greater 
or less amount of yolk for its nutrition. The spermatozoén, 
on the other hand, is relatively minute and capable of locomotion. 
It contains no food substances, and only sufficient protoplasm 
to serve as transmitter of paternal qualities and for organs 
of locomotion. 
The Spermatozoon. The spermatozodn (Fig. 1) is an elon- 
gated flagellated cell in which three main divisions are distin- 
guished, viz., head (caput), neck (collum) and tail (eauda). The 
head contains the nucleus, and the neck the centrosomes of the 
sperm mother-cell or spermatid. The tip of the head is often 
transformed into a perforatorium. Three parts may be recog- 
