AL TERN A TION OF GENERA TIONS. 



195 



offspring is virtually continuous, either in itself or through its nucleus, 

 with the ovum which gave rise to the parent. A chain of ovum-like 

 cells is only demonstrable in a few cases; but Weismann overcomes 

 this difficulty by supposing that what really keeps up the protoplasmic 

 tradition or continuity between the parental ovum and the next 

 generation is a specific and stable portion of the nucleus — the "germ- 

 plasma." When a medusoid goes off from a hydroid, it carries with 

 it a legacy of this germ-plasma, continuous with that which gave rise 



Fig. 75. — The hermaphrodite fern prothallus contrasted with (2 a) the male and (2 b) 

 the female thallus of liverwort, and (3 a and /') male and female prothal- 

 lus of horsetail. Above are the corresponding reductions of the sexual 

 prothallia in (4) Salvinia, (5) Isoetes, (6) Cycad and Conifer, and (7) 

 Phanerogam. 



to the hydroid. This legacy forms the reproductive elements of the 

 medusoid, which in turn give rise to hydroids. The medusoid itself 

 is a modified asexual growth, into which some of the germ -plasma of 

 the hydroid has migrated ; it is literally only the bearer of the hydroid 

 germ-plasma. Weismann' s classic researches on hydroids have 



