686 , The American Naturalist. [July, 
Part IV of the valuable Bibliography of America Economic Entomo- 
logy has been recently issued by the Department of Agriculture. It 
includes authors from A to K, and shows the same careful compilation 
by Dr. Samuel Henshaw as the previous issues of the series. 
An important Report upon the Parasitic Hymentoptera of the Island 
of St. Vincent by Messrs. Riley, Ashmead and Howard has recently 
been issued by the Linnzan Society (Journal Zoology, XXV, pp. 55- 
254). The material was collected by Mr. H. H. Smith, and contained 
six new genera and 299 new species. 
EMBRYOLOGY. 
Origin of Twins.—Jacques Loeb of the University of Chicago 
contributes to the fourth part of Roux’s new peroidical—Archiv. für 
ntwickelungsmechanik der Organismen—an illustrated article in 
which the results of his experiments upon echinoderm eggs are set forth 
along with a hypothesis of the mechanical origin of double embryos. 
He found that when the eggs of the sea-urchin *“ Arbacia” were put 
into water less salt than normal the membrane might burst as if from 
osmotic pressure and part of the egg protoplasm ooze out from the rent. 
In case this extruded part remained in continuity with the rest of the 
egg farther development might result in the formation of a double 
larva. 
Many most interesting double and triple larve so produced are 
figured with the abnormal skeletal structures seen in them. 
The author then adopts the ideas of Quincke in an attempt to explain 
the production of double monster in general and in the higher animals 
in special. 
Quincke regarded certain protoplasm movements as similar to those 
of oil and water when mixing in the presence of soda or of albumen. In 
such cases more or less violent “extension currents” are produced: 
currents which Biitschli would assume in the movements of the pseudo- 
podia of an ameeba on his hypothesis that protoplasm has a vescicular 
structure. 
Professor Loeb assumes that mechanical currents are normally pres- 
ent in the process of cleavage and that in the abnormal process of 
double formation there is, for various unknown reasons, an exagger- 
