On Reproduction, Animal Life Cycles. 
85 
most of the higher groups, as was argued earlier, but with racial ad- 
vance those particular ones that are to become sexually reproductive 
in the individual that contains them get arranged into special germ 
glands or gonads. In those animals higher than the Coelenterata 
that have no large coelomic cavities these gonads are small sacs, con- 
sisting only of germ cells proper with the various kinds of nurse 
cells already mentioned. In animals higher than these a greater 
state of division of labor has taken place between the germ cells : 
the gonads are in the form of large sacs, known as coelomic sacs, and 
the cells of the germinal cycle have differentiated not only into 
germ cells proper and their nurse cells but also into peritoneal cells 
and the various derivatives of the latter. This is an application of 
the gonocoel theory to the germinal cycle in general. This theory, 
originated by Hatschek 13 , Bergh , 14 and particularly Eduard Meyer , 15 
regards the coelomic sacs of the higher animals as comparable with 
the gonads of the lower, and the latter, therefore the secondary meso- 
blast, as a composition of genital cells. It is one of the most lucid 
and comprehensive of morphological theories, and probably correct; 
at least there is little evidence that speaks against it. 
The reason for mentioning this idea here is to show that in the 
multicellular body three main kinds of cells are to be distinguished 
with regard to their origin. These are: the definitive germ cells, 
those that pass through the whole germinal cycle; the somatic cells 
proper, those that early separated themselves from that cycle; and 
a third, somewhat intermediate, group of cells that start out along 
the path of the germinal cycle, but later lose the ability to reproduce 
the whole individual and become nurse and follicular cells as well as 
cells of the peritoneum and its derivatives. That is, what are com- 
monly designated somatic cells, include besides somatic cells proper 
cells that are intermediate in character between these and germ cells 
proper. 
This thought followed out comes to supplant, or rather modify, 
the earlier germ layer theory of Huxley and Kowalevsky. That con- 
cluded that in the ontogeny of the Metazoa, two primary layers are 
produced, the ectoblast and entoblast, and that in all ectoblast is homol- 
ogus with ectoblast and entoblast with entoblast. This theory neglects 
the germ cells, on that account comes the need for the following modi- 
fication. All Metazoa develop two primary somatic layers and besides 
these a germinal layer, secondary mesoblast if you will, whose cells 
13 Embryonalentwicklung und Knospung der Pedicellina echinata, 1877, Zeit. 
wiss. Zool., 29. 
14 Die Excretionsorgane der Wuermer, 1885, Komos, 17. 
15 Studien ueb'er den Koerperbau der Anneliden, 1901, Mittheil. Zool. Stat. 
Neapel, 14. 
