212 CONTINUITY OF THE GERM-PIASM AS THE [IV. 



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 an actual migration of primitive germ-cells from 

 the place where they arose to the position at which they 

 undergo difierentiation 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 \ 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 position have always taken 

 place by very gradual stages, and never suddenl3% 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^ 

 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 primi- 

 tive germ-cells, which are given in my work already referred 

 to. The instance of Obelia is of especial interest as the 

 direction of displacement is here reversed, taking place cen- 

 trifugally 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 



[} I adopt this term, suggested by E. Ray Lankcstcrand G. C. Bourne, 

 as the name of the supporting lamina of Coelentcrata. See 'Quart. 

 Journ. Microsc. Sci.' Jan 1887, p. 28. — E B. P.] 



- Dr. Clemens Hartlaub. ' Ueber die Entstehung der Sexualzellen bei 

 Obelia.' Freiburg. Inaugural Dissertation : see also ' Zeitschrift fiir 

 wissenschaftliche Zoologie,' Bd. XLI. 1884. 



