24 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
The groove becomes a tube and passes inwards. The gonad and the 
secondary duct of the male become connected with the primary duct 
before birth, while the gonad of the fresh-born female is not connected 
with the primary duct. For some time after birth the gonad of both 
sexes has a quite indifferent character ; the efferent ducts become so far 
histologically differentiated that their epithelium is ciliated, while that 
of the gonad has no cilia. The derivation of the gonad from the peri- 
cardium is a further proof that the latter corresponds to the secondary 
coelom. 
Planorbis has a pericardium with a distinct septum before the heart 
is formed. While the renal organ is mesodermal in origin its duct is 
formed by an invagination of the ectoderm of the rudimentary mantle- 
cavity. Sections reveal the presence of an outer and an inner opening 
to the pi'imitive kidney. The inner, which leads into the coelom, is not 
terminal, but set laterally on the ciliated portion, at the end of which is 
a group of small cells indistinctly separated from one another. While 
that part of the primitive kidney which leads to the exterior corresponds 
to the large central cell, the ciliated internal part is formed of several 
cells, the nuclei of which are distinctly visible. 
Development of Bythinia tentaculata.* — Dr. R. v. Erlanger has 
come to conclusions very different from those of Dr. P. Sarasin and in 
agreement with what he has already observed in the allied Paludina. 
Segmentation is of the normal Gastropod type, and most like that of 
Planorbis and Neritina. The hindermost macromere may be known as 
the endo-mesoderm-cell, for it divides into two, one of which retains the 
place of the hinder macromere, while the other is pushed nearer to the 
animal pole. This latter divides into two, but in the transverse direc- 
tion ; the cells thus formed are the primitive mesoderm-cells, which lie 
on either side of the long axis. 
The germ becomes flatter and flatter, and has the form of a spherical 
triangle, with the apex directed forwards. The blastopore appears as a 
long slit which occupies the whole length of the ventral surface ; the 
archenteron forms a pretty wide cavity ; the permanent mouth is derived 
directly from the blastopore. There now appear the first signs of a 
velum in the form of a double row of clear ciliated ectodermal cells, 
which form a circlet set obliquely to the longitudinal axis. The meso- 
derm has by this time become bilaminate, and forms to the right and left 
a saccule ; these pas6 into one another at the hinder end, and gradually 
grow out forwards and dorsalwards. The coelom between the two layers 
can be easily seen. 
The archenteron alters in form, becoming much narrower posteriorly. 
On the dorsal surface of the hinder end the shell-gland appears as a 
thickening of the ectoderm, and at the same time the foundations of the 
cerebral ganglia appear as lateral thickenings of the velar area. At 
this stage the glandular part of the primitive kidney appears as a clump 
of mesoderm-cells. At the hinder end of the blastoporal groove a small 
pit may be noticed, which marks the point at which the anus appears 
later on. 
The oesophagus is formed at the point where the mouth was formed 
* Zool. Anzeig., xiv. (1891) pp. 385-8. 
