252 M.Van Beneden on the Physiology of the Simple Ascidians.  - 
first glance, it becomes easy of comprehension by a study of some 
of the lowest animals, and its solution might have been given 
long ago. The Hydre are reproduced in the same way during 
the summer, without the assistance of males, and are viviparous 
during several generations which follow each other successively 
until the approach of winter; then, instead of buds, eggs appear, 
and we observe, at the same time, spermatozoides, representing 
the male organ, which fructify the eggs that are to preserve the 
species until the following spring. Is it not the same pheno- 
menon in the Aphides? and, to give an explanation of it, is it not 
sufficient in fact to say that they are gemmiparous throughout 
the summer, and have consequently no need of the male element ? 
All that appears to me surprising here is to see this double mode 
of reproduction, so common in several of the lowest animals, in 
animals so high in rank as the Articulata. ) 
I have satisfied myself that in the egg of the Ascidia, as every- 
where else, there are the two vesicles of Purkinje and of Wagner. 
The former only had been hitherto noted. 
The manner in which the blastoderm is formed is a point 
of the highest interest in the history of embryonic develop- 
ment. At first the vitellus runs through the same phases as 
in other classes, viz. it divides itself into lobules which become 
small and smaller, and which have each a clear and transparent 
vesicle im their centre: we may say there are so many individual 
vesicles of Purkinje. This phenomenon has also escaped the 
notice of my predecessors. After this change in the vitellus 
the blastoderm appears. Is the blastoderm then formed, as in 
the superior animals, upon a determinate point, whence it extends 
slowly over the whole vitellus, or rather is it formed simulta- 
neously upon all the points without forming a disc? I believe 
the latter view is the correct one, but the former is adopted by 
my predecessor in this matter. It has always seemed to me that 
the blastoderm appears at once upon every point of the surface 
of the vitellus ; and that it constitutes, from the moment of its 
appearance, a continuous membrane without any aperture. 
The caudal appendage of the tadpole of the Ascidia, instead of 
being formed, as has been pretended, by separation, is developed 
by extension, in the same way as appendages in general. We 
have seen nothing in these Ascidi@ that resembles the zigzag that 
has been figured in that caudal appendage. 
Other appendages are formed-on the side opposite to the tail, 
but these are constant neither in their number nor in their re- 
spective positions. They have been called suckers, but I have 
seen nothing to justify this designation. The embryo is affixed 
by its mteguments, and these presumed suckers are often not 
even long enough to reach the exterior envelope. 
