208 THE CONTINUITY OP THE GERM-PLASM AS THE 



localized area from its original position (the manubrium of the 

 Medusa) in a centripetal direction. For the purposes of the present 

 enquiry it is unnecessary to discuss the reasons for this change of 

 position. The phyletic displacements of the localized areas are 

 brought about during ontogeny by^n actual migration of primitive 

 germ-cells from the place where they arose to the position at which 

 they undergo differentiation into germ -cells. But we cannot believe 

 that primitive germ-cells would migrate if the germ-cells could 

 be formed from any of the other young cells of indifferent character 

 which are so numerous in Hydroids. Even when the localized area 

 undergoes very slight displacement, e. g. when it is removed from 

 the exterior to the interior of the mesogloea 1 , the change is always 

 effected by active migration of primitive germ-cells through the 

 substance of the mesogloea. Although the localized area has been 

 largely displaced in the course of phylogeny, the changes in posi- 

 tion have always taken place by very gradual stages, and never 

 suddenly, and all these stages are repeated in the ontogeny of all 

 existing species, by the migration of the primitive germ-cells from 

 the ancestral area to the place where the germ-cells now arise. 

 Hartlaub 2 has recently added a further instance (that of Obelia] to 

 the numerous minute descriptions of these phyletic displacements of 

 the localized area, and ontogenetic migrations of the primitive germ- 

 cells, which are given in my work already referred to. The 

 instance of Obelia is of especial interest as the direction of dis- 

 placement is here reversed, taking place centrifugally instead of in 

 a centripetal direction. 



But if displacements of the localized areas can only take place by 

 the frequently roundabout method of the migration of primitive 

 germ-cells, we are obliged to conclude that such is the only manner 

 in which the change can be effected, and that other cells are unable 

 to play the role of the primitive germ-cells. And if other cells are 

 unable to take this part, it must be because nucleoplasm of a 

 certain character has to be present in order to form germ-cells, or 

 according to the terms of my theory, the presence of germ-plasm is 



[ J I adopt this term, suggested by E. Ray Lankester and G. C. Bourne, as the 

 name of the supporting lamina of Coelenterata. See ' Quart. Journ. Microsc. Sci.' 

 Jan. 1887, p. 28 . E. B. P.] 



a Dr. Clemens Hartlaub, ' Ueber die Entstehung der Sexualzellen bei Obelia.' 

 Freiburg, Inaugural Dissertation : see also 'Zeitschrift fur wissenschaftliche Zoologie.' 

 Bd. XLI. 1884. 



