ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
697 
(as Shipley expressed it), but arises from a folding of the single- 
layered ectoderm of such a nature that the sides of the fold remain in 
contact. The teloblast seems like a reserve for both ectoderm and 
endoderm, as it comes into intimate relations with organs which arise 
from both layers ; but in P. planeri it certainly does not give origin to 
the mesoderm. It is comparable to the “sickle” which Kupffer has 
described in other Vertebrate embryos. 
In the second period, the nerve-cord is separated from the ectoderm, 
the gnt becomes differentiated, and the mesoderm appears. There is a 
striking want of symmetry in the neural strand at the region of transi- 
tion from head to trunk. But of most interest during this period is the 
endoderm in its two portions, the epithelial wall of the gut, and the 
yolk-cells which function as reserve-material. For now the formation 
of the dorsal mesoderm and of the segmental plates sets in. In the 
head-region, where the gut is not surrounded by the yolk-cells, the 
endoderm forms hollow dorsal folds in true enterocoelic fashion. But 
in the trunk-region the massive cushions of yolk-cells, lying on each 
side of the axial organs, change gradually into mesoderm, acquire a 
coelom cavity, and are separated from the ventral mass. Yet this appa- 
rent combination of enterocoelic and schizocoelic processes is not strange ; 
it depends upon the absence or presence of these endodermic yolk-cells. 
The open mesodermic fold of the gut in the head-region passes gradually 
with the thickening of its walls and the obliteration of its cavity into 
the massive mesodermic cushion of the trunk. 
At this stage in his memoir Kupffer ceases to sketch the history of 
each period in its entirety, and proceeds to trace the rise of the various 
systems. The remainder of the present paper is devoted to the nervous 
system and sense-organs. As concerns the peripheral system, Kupffer 
recognizes at the end of the fourth period five distinct rudiments, which 
subsequently come into connection: — dorso-spinal and ventral-spinal 
nerves, lateral ganglia, branchial nerves, and epibranchial ganglia. 
£. Histology-* 
Intra- and Inter-cellular Ducts.f — Prof. F. Leydig supports the 
opinion which he expressed many years ago that the roots of a duct 
proceeding from a cell are continued into spaces lined by sjDongioplasm, 
and wonders, not unnaturally, that a recent observer of the intracellular 
nature of the ducts in the nephridia of the leech has not appreciated the 
figure in the well-known ‘ Histologie ’ of 1857. Some recent observa- 
tions on the exploding glauds of Brachinus (along with which Agonum 
must be ranked as another “ bombardier-beetle ”) have convinced the 
veteran histologist that efferent canals may arise from intercellular 
ducts, and he recommends the study to those interested. 
Cell-Studies.:]: — In the present communication Dr. T. Boveri deals 
with the relation of the chromatic nuclear substance in the formation of 
the polar globules and in fertilization. From the study of various 
animals he is led to certain conclusions, which may be thus summarized. 
* This section is limited to papers relating to Cells and Fibres. 
f Biol. Centralbl., x. (1890) pp. 392-6. 
X Jenaische Zeitschr. f. Naturwiss., xxiv. (1890) pp. 314-101 (3 pis.). 
