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 w^hole. 

 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, etc. 



V. General Character of Germ-cells 



As already remarked the ovum and spermatozoon 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 spermatozoon, 

 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 spermatozoon (Fig. 1) is an elon- 

 gated flagellated cell in which three main divisions are distin- 

 guished, viz., head (caput), neck (collum) and tail (cauda). 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- 



