708 ; The American Naturalist. [August, 
EMBRYOLOGY: 
The Development of Paludina vivapara.’—R. von Erlan- 
ger contributes two papers on this subject which form a comprehensive 
and valuable study of the development of this species. He describes 
concisely and clearly the development of the tissues and organs, giving 
special attention to the origin of the mesoderm and to the formation 
of the pericardium, heart, primitive kidneys, permanent kidneys, renal 
ducts, reproductive organs, and nervous system. The author had an 
abundance of material and believes he was able to fully verify all his 
results. Heretofore no one has succeeded in keeping alive the embryos 
after removing them from the opaque albuminous capsule which 
encloses them. The author found they would live for a timein a solu- 
tion of 20cc. of egg albumen, 1g. of common salt and 200cc. of 
water; so that he was able to observe the processes of development in 
the living embryo. Of the fixing agents used Kleinenberg’s picro sul- 
phuric acid with a drop of 5% osmic acid added was by far the most 
successful. 
The mesoderm arises from the archenteron at the time of the forma- 
tion of the velum. The ventral wall of the archenteron pushes out as 
a single large sac, which soon pinches off from the rest of the ento- 
erm. For a time it has the form of a closed vesicle lying in the ven- 
tral half of the embryo between the ectoderm and entoderm. This 
-vesicle enlarges, its cells becoming somewhat flattened in the process. 
Later the walls of the vesicle break up into their constituent cells, the 
disintegration beginning at the mid-ventral point of the vesicle. Some 
of the cells apply themselves to the ectoderm (somatopleure), some to 
_ the entoderm (splanchnopleure), others, star shaped, form an extensive 
net-work filling the segmentation cavity ; processes from the somato- 
pleuric and splanehnopleuric mesoderm cells also join the net-work. 
The different organs now begin to make their appearance. The 
coelom of Paludina, according to this description, arises as a single, 
~ median, ventral evagination from the archenteron. 
At the time when the rudiment of the stomadæal invagination 
appears there can be seen in the posterior part of the embryo, below 
This department is edited by Dr. E. A. Andrews, Johns Hopkins University. 
ees seh eect der Paludina vivapara. R. von Erlanger, Morph. Jahrb., vol. 
vii, 1891; Part I, August; Part II, October. ya 
