THE GERM-PLASM THEORY 415 



manubrium of the medusoid, which is still present even when great 

 degeneration has occurred, or even— in the most extreme cases of 

 degeneration — into the ectoderm of the brood-sac. This is the case 

 in the genus Sudendrium, of which Fig. 95 gives a diagrammatic 

 representation. 



The most interesting feature of this migration of the germ-cells 

 is that the cells invariably arise in the ectoderm {hz""), then pierce 

 through the supporting lamella {st) into the endoderm {hz'"), and 

 then creep along it to their maturing-place. Once there they break 

 through again to the outer layer of cells, the ectoderm [hz), and come 

 to maturity {Ei). That they make their way through the endoderm 

 is probably to be explained by the fact that they are there in direct 

 proximity to the food-stream which flows through the colony {GH = 

 gastric cavity), and they are thus more richly nourished there than 

 in the ectoderm. But although this is the case, they never arise in 

 the endoderm ; in no single case is the birthplace of the germ-cells to 

 be found in the endoderm, but always in the ectoderm, no matter 

 how far back it may have been shunted. Even when the germ-cells 

 migrate through the endoderm, their first recognizable appearance 

 is invariably in the ectoderm, as, for instance, in Podocoryne and 

 Hydradinia. The course of affairs is thus exactly what it would 

 necessarily be if our supposition were correct, that only definite 

 cell-generations — in this case the ectoderm-cells — contain the complete 

 germ-plasm. If the endoderm-cells also contained germ-plasm it 

 would be hard to understand why the germ-cells never arise from 

 them, since their situation ofiers much better conditions for their 

 further development than that of the ectoderm-cells. It would also 

 be hard to understand why such a circuitous route was chosen as 

 that exhibited by the migration of the young germ -cells into the 

 endoderm. Something must be lacking in the endoderm that is 

 necessary to make a cell into a germ-cell : that something is the " 

 germ-plasm. 



If we accept the theory of the continuity of the germ-plasm as 

 in the main correct, it appears that higher animals and plants are 

 constructed of two kinds of elements, the somatic cells and the germ- 

 cells ; both owe their being to the germ- plasm of the ovum, but the 

 former do not contain it complete but only in individual determinants ^ 



' Boveri has recently made an observation upon the thread-worm of the horse, 

 which points to the correctness of the conception of the germ-plasm. The two first 

 segmentation-cells both receive the four chromosomes of the spocies, but, in one of the 

 two, a portion of the chromatin breaks off and degenerates, or dissolves, at least as far 

 as can be seen. The other cell retains the whole mass of chromatin, and from this 

 there arise later the primitive genital-cells. In the germ-track, therefore— so we must 



