485 



real beginning of the building-up of the embryo must be taken as 

 dating. It and its cells can no more revert to this starting point, than 

 a man can return to his childhood. The life of the individual has 

 been initiated. 



While all this is taking place, the germ-cells have remained 

 passive. If one or other of them come into activity, the result is the 

 abnormal production of a twin, or of a pathological groM'th or dermoid, 

 an abortive embryo. Anon the time arrives for the awakening of the 

 primary germ-cells, and for the formation of secondary germ-cells. 

 They begin to increase, but this multiplication is such, that no diffe- 

 rentiations, comparable to those which produced the embryo, result. 

 Instead thereof, a much greater number of secondary germ-cells arises. 

 Probably with the new divisions the faculty of developing spontaneously 

 becomes lost. At any rate, they have entered upon a portion of a 

 life-cycle, leading directly to a new conjugation of gametes, and most 

 certainly they have evaded the line, ending in a cul-de-sac, the form- 

 ation of an embryo. 



If one examine the life-cycle of the germ-cells from any con- 

 jugation to a succeeding one, leaving out of consideration the form- 

 ation of a larva or of an embryo, it is obvious, that in all its details 

 it resembles closely the life-cycles of many Protozoa. 



Conjugation of two unlike gametes is followed by a number of 

 cell-divisions, the products, the primary germ-cells, containing the 

 duplicated number of chromosomes. Then comes a resting phase, 

 conditioned by the environment within the developing embryo. By 

 and bye, there are new divisions, still with duplicated chromosomes, 

 the products being the secondary germ-cells, and these mitoses culmin- 

 ate, as in many Protozoa, in certain divisions, accompanied by a 

 reduction of chromosomes, just prior to a new conjugation. 



Indeed, the germ -cells may be regarded as unicellu- 

 lar organisms, which pass one part of their life-history 

 within a multicellular sterilised stock, the embryo, or 

 Metazoon, formed by one of them at a definite period 

 in the life- cycle. 



Necessarily from this point of view the immortality, postulated 

 by Weismann for the Protozoa and for the germ-cells of the higher 

 animals, but denied by him to the Metazoa themselves, attaches to 

 the germ-cells, and not to the embryo. 



In this way the continuity of a hypothetical germ-plasm resolves 

 itself into an actual morphological continuity of germ-cells. 



