54 SUMMARY OF CURRENT RESEARCHES RELATING TO 
fourth endoderraic pouches, whose dorsal walls form the thyroidean 
glandules. These lateral thyroids separate from the pharynx and from 
the thyroidean glandules, and fuse with the median thyroid to form the 
final thyroid gland. 
The lobes of the thymus are primitively represented by two tubes 
formed at the expense of the third endodermic pockets. They have a 
somewhat convex history. 
The thymic glandules arise on the wall of the third endodermic 
pouches, and have a structural development like that of the thyroidean 
glandules. 
Pleural Cavities and Pleuro-Pericardial Membranes. *— Dr. A. 
Brachet has tackled this difficult subject from the embryological side. 
We have only space to notice two general conclusions, which may suggest 
to those interested the importance of the paper. The diaphragm is 
formed by the septum transversum, and two of its extensions — the pleuro- 
peritoneal and the peritoneo-pericardial membranes. The pleuro-peri- 
cardial membrane properly so-called is formed, in small part, by the 
quite anterior portion of the septum transversum , and, in great part, by 
the pleuro-pericardial extension of this septum. 
Development of Pronephros and Mesonephros in Myxine.j — Herr 
0. Maas furnishes an important contribution to tha comparative study 
of the excretory systems of the lower Yertebrates. He was fortunate 
in obtaining unusually small specimens of Myxine (one of only 8*5 cm.), 
and employed them for the investigation of the development and rela- 
tions of the pronephros and mesonephros, recently the subject of keen 
discussion between Spengel and Semon. His interpretation of his 
observations is as follows : — The pronephros consists primitively of a 
series of tubules, segmentally arranged, which in the anterior region 
open into the pericardium by one or several funnels, and which have 
a superficial network of capillaries formed by the dividing up of vessels 
from the dorsal aorta. The general similarity to Boveri’s tubules in 
AmpMoxus hardly requires emphasis, but a marked difference is dis- 
cernible in the absence of external openings, and in the tendency of 
the blind ends of the tubules to fuse together. These pronephric 
tubules, hypothetically extending throughout the body, undergo dif- 
ferent modifications in (1) the region of the true (adult) kidney, in (2) 
the intermediate zone, and in (3) the head-kidney of the adult. (1) In 
the posterior region of the body the internal openings are lost, the 
outer ends fuse together to form the pronephric duct, which however 
retains some excretory significance, and the segmental capillary net- 
works persist on the surface of the duct. The pronephric tubules of 
this region, though in a modified form, are thus functional throughout 
life. (2) In the intermediate zone the tubules undergo degeneration,, 
but in young forms traces of tubules or of the pronephric duct are 
readily perceived. (3) In the region of the head-kidney the outer 
ends of the tubules fuse together and with part of the capillary net- 
work to form a mass of lymphoid tissue — the adrenal. Certain portions 
of the capillary network persist and become specialised to form the 
* Journ. Anat. Physiol , xxxiii. (1897) pp. 421-60 (2 pis.). 
f Zool. Jahrb., x. (1897) pp. 473-510 (4 pis.). 
