43 



becomes cells a.h. and i.p^, each containing but half the 

 characteristics — determinants — of a. p., and so on, until the 

 determinants are all distributed. The products of a.p^ are 

 known to Weismann as idioplasm. The germ-plasm and 

 idioplasm combined constitute the embryo. In course of 

 development the germ-plasm ultimately reaches the germinal 

 epithelium, and there initiates the primordial germ-cells. 



The other aspect — method (b) — of the history of the 

 germ-cells from one generation to another is set forth in 

 diagram No. 2. It is here supposed that no doubling 

 division whatever takes place, but that qualitative halving — 

 differentation — commences with the first division. In this 

 method it must be assumed that the primordial germ -cells 

 are provided for by certain determinants, in exactly the same 

 way as for eyes, lung tissue, etc. 



It matters little which of these views we accept — both 

 may be assumed — for in each case continuity of germ-phtsni 

 is provided for. Really, however, neither of these views has 

 any support in the actually observed facts. 



Weismann writes : — " The transmission of the germ- 

 " plasm from the ovum to the place of origin of the 

 " reproductive cells takes place in a regular manner, through 

 " perfectly definite series of cells which I call genn tracks 

 " These are not actually recognisable, but if the pedigree oi 

 " the cells in the embryogeny is known they may be traced 

 " from their termination to the germ-cells backwards to the 

 " ovum. 



" This assumption is supported by the fact that a direct 

 " or, at any rate, very close connection can be proved to exist, 

 " although only in rare instances, between the germ-cells of 

 " two consecutive generations. In the Diptcra the first 

 '• division of the egg-cell separates the nuclear material of the 

 " subsequent germ-cells of the embryo from that of the 

 " somatic cells, so that in this case a direct continuity can 

 " be traced between the germ-plasm in the germ-cells of the 

 " parent and offspring." " 



• Weismann : '"The C!crm-pl;isni, " p. 184. 



