252 M. Van Benedeii 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 Hydra 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 phseno- 

 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 Ascidice, 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 in their centre : we may say there are so many individual 

 vesicles of Purkinje. This phsenomenon 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 AscidicB 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 integuments, and these presumed suckers are often not 

 even long enough to reach the exterior envelope. 



