476 Prof. Stein on the Development and 



sent no clear cilia, but are seated upon an excessively fine, non- 

 contractile stalk, and which, when they become free, present ex- 

 actly the same form and movements as the embryos which have 

 proceeded from the maternal vesicle. It is therefore perhaps as 

 good as certain, if I assume that the embryos fix themselves 

 soon after they are born, particularly as I frequently saw such 

 young Vorticellce again in my infusion soon after the first burst 

 cysts were observed. The production of numerous monad-like 

 embryos then, forms unmistakeably the concluding phase in 

 the course of development of the Vorticellce, and this course 

 would be very simple were not the Acinetce interposed in the 

 cycle. 



How shall we now interpret this last phase ? Two circum- 

 stances throw light upon it. First the Acinetce set free their 

 whole germ-nucleus as a ciliated individual, which, except in the 

 want of a stalk, exhibits the whole organization of a Vorticella, 

 and is in no respect different from an individual which has arisen 

 by budding. The Acineta, therefore, is properly nothing else 

 than a Vorticella, which out of the still, pupa-condition, has re- 

 turned into active existence, but under an altered form. It sends 

 from the surface of its body delicate radiating processes, which 

 have independent movements, and not merely, as I previously 

 believed, serve to keep off enemies, but also certainly take in 

 nuti'iment by their surfaces, though naturally only in a fluid 

 form. That the Acinetce can be nourished from without seems 

 to be a necessary conclusion, from the fact that I saw the germ- 

 nucleus change into an embryo only in Acinetce of a certain size; — 

 not in the very small Acinetce, which therefore must grow and 

 be nourished from without. The production of Vorticellce by the 

 intermediation of Acinetce then is perfectly equivalent to gem- 

 mation ; it is a kind of internal gemmation ; whilst propagation 

 by the change of the whole inner encysted body of a Vorticella 

 into numerous embryos is to be regarded as the equivalent of 

 the sexual propagation of the higher animals. In this manner 

 there is a sort of alternation of generations in the Vorticellce, 

 though perhaps not in so marked a form as in other Inverte- 

 brata ; since each early phase of development may change into 

 the final phase of the whole series, under peculiar conditions ; — 

 what these conditions are, however, I have not been able, as yet, 

 clearly to determine. The embryo may, under certain circum- 

 stances, become encysted after a short period of existence, as the 

 very small cysts, often to be observed, demonstrate. Further, a 

 bud just freed from its parent may at once become encysted, and 

 the stalked Vorticellce are able to become so at all stages of their 

 growth. The cysts, from the smallest to the middle-sized, ap- 

 pear to be able to pass into the Acineta-form only; from the 



