ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
593 
from the germinal streak and the mesoderm into the yolk, but these 
speedily undergo degeneration and do not become yolk-cells. The 
mesoderm has a variable origin, and may originate in different ways in 
the different parts of the same embryo. 
In a discussion of the theoretical bearing of these results, the author 
draws the following conclusions from his own observations and those 
of others on the endoderm of insects. The yolk- cells represent the 
true endoderm (Heymons, Lecaillon) ; in Campodea they are directly 
converted into the epithelium of the mid-gut (Heymons) ; in Lepisma 
and the Odonatse part form the mid-gut and part degenerate ; in the 
Lepidoptera and other higher insects the yolk-cells (endoderm) wholly 
degenerate and the ectoderm takes on their function ; in the Apliidse 
the yolk-cells are few in number (Will), so that the endoderm is greatly 
reduced ; finally, in some parasitic Hymenoptera there is neither yolk 
nor yolk-cells (Koulaguine), so that the endoderm is entirely absent. 
It would thus appear that there are organisms without any true inner 
layer, so that it cannot be said that an intrinsic physiological distinction 
exists between ectoderm and endoderm. The method of origin of the 
yolk-cells in some insects is such as to make it extremely difficult to 
apply the usual scheme of gastrulation. Both on physiological and 
morphological grounds, therefore, the author doubts the possibility of 
homologising the germinal layers throughout the animal kingdom, and 
believes that in the study of the phylogeny of the Metazoa the value 
of the germ-layer theory has been overestimated. 
The investigation was carried out chiefly on eggs of Lasiocampa 
fasciatella Men. var. excellens, but other material was used for com- 
parison. 
Variation in Butterflies.* — Prof. J. W. Spengel describes, under 
the name of “ aberrations,” a number of forms of Papilio machaon, in 
which the deviation from type seems to be greater than that in “ normal 
variations.” The examples described are both such as have been found 
in natural conditions and such as are produced by experimental breed- 
ing. One of the most interesting cases is the aberr. nigrofasciata , 
which occurs in natural conditions from unknown causes, in autumn 
from broods kept under the influence of very low temperatures, in 
spring from broods subjected to no artificial conditions of temperature. 
No general conclusions are drawn, but the evidence seems to be against 
the view that the temperature acts directly in producing deviation from 
type in butterflies. 
Testis of Hydrophilus.f— Herr C. de Bruyne has studied the histo- 
logy of this organ with special reference to the follicular cells of the 
seminiferous tubules. Each tubule contains a number of these folli- 
cular or enveloping cells which surround the spermatogonia. Both 
follicular cells and spermatogonia originate from the primitive germinal 
epithelium, but the function of the follicular cells is merely to protect 
and nourish the developing spermatozoa. When the sperms are mature 
the follicular cells undergo degeneration and break down. In the ovary 
of insects in general there are similarly two kinds of cells, the folli- 
* Zool. Jahrb. (Abt. Syst.), xii. (1899) pp. 337-84 (3 pis. and 5 figs.). 
t Anut. Anzeig. (Erg'anzungsheft), xvi. (1899) pp. 115-23 (4 figs.). 
