436 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
In other Urodela the pronephros consists, with two exceptions, of 
only two canals, each with a single nephrostome. In Amphiuma the 
pronephros is in relation with three primitive segments. There is but 
one true pronephric canal for each segment. Field’s observations led 
him to decide against regarding the pronephros as a definite ancestral 
organ distinct from the mesonephros. 
In IchthyopJiis , which has a pronephros related to 12 or 13 segments, 
the pronephros and the mesonephros are in contact, or even superposed. 
The pronephros of the other Batrachians which have been studied is 
followed by an interval of 2-4 segments without an excretory canal, 
before one reaches the mesonephros. In Ampliiuma the interval extends 
to 22 segments. 
Development of the Thymus.* — Mr. J. Beard has had an opportu- 
nity of studying this in an extensive series of embryos of Baia batis. 
There seems to be little doubt that the organ is, in the main, hypoblastic 
in nature, and Mr. Beard has evidence that the thymus is actually a 
portion of the lining of each gill-cleft, but there are no thymus-elements 
in connection with the spiracle or the mouth, though the author inclines 
to the view that the vesicular follicle of the spiracle and of the angle of 
the mouth are rudimentary equivalents of such elements. 
It has been found that the ordinary teaching that the cell-elements 
of the thymus retain their epithelial characters after their proliferation 
is completed, is incorrect ; the original cells of the thymus are some- 
what rounded bodies possessing more the characters of lymph or adenoid 
cells than of epithelium. 
The migration of lymph-cells into the thymus is generally accepted, 
Kolliker alone having given a true account of the facts ; he showed that 
the thymus cells become at first arranged in epithelial fashion, forming 
glandular-like tubes ; in Baia and other fishes the similar cells, immedi- 
ately on their birth from the parent epithelium, take on the lymphoid 
characters which are only later acquired by Mammals. All Mr. Beard 
has seen leads him to deny the migration of lymph-cells into the thymus 
at any period ; he considers that the lymph- elements of the thymus are 
the direct offspring of the epithelial cells of a gill-cleft. In other words, 
a leucocyte-forming structure arises from the hypoblastic epithelium of 
gill-clefts. 
This formation of leucocytes in the immediate neighbourhood of the 
gills must be for the protection of the gills themselves ; it is suggested 
that they may serve to devour and remove parts of the gills which have 
undergone necrosis, while their importance as guards against microbes 
need not be dilated on. To Mr. Beard the thymus in its function bears 
some resemblance to the tonsils, which, in the higher Mammals, where 
respiration is on a different type, usurp their duties. 
Haacke’s Theory of Gemmaria.f — Prof. R. von Lendenfeld sum- 
marizes and criticizes this theory, recently elaborated by Dr. W. Haacke4 
The plasma of the egg-cell or any cell consists of the smallest organized 
* Anat. An^ig., ix. (1894) pp. 476-86. 
t Biol. Centralbl., xiv. (1894) pp. 413-6. 
X ‘ Die Schopfung der Thierwelt,’ Leipzig, 1893; ‘Gestaltung und Vererbung,’ 
Leipzig, 1893. 
