Anatomy. — "The egg-cleavage of Volvox globator and its relation 

 to the movement of the adult form and to the cleavage types 

 of Metazoa." By Dr. H. C. Dklsman. (Cominiinicated by Piot'. 



J. BOEKEj. 



(Comtnunicated in the meeting of June 29, 1918). 



For the zoologist still more than for the botanist Volvox is an 

 interesting object. Already in this organism, where it is still dubious 

 whether we have to consider it as a plant or as an animal, we see 

 indicated the main lines along which the piiylogenetic development 

 of the Metazoa has taken its course. Bütschli.^) rightly observes 

 that Volvox is no longer to be considered as a colony of Protozoa, 

 but as a plnricellular organism of simple structure. Not only do the 

 cells communicate with each other by plasmodesms, forming thus 

 one single mass of protoplasm, but also there is a difference between 

 mortal somatic and potentially immortal propagation cells as is 

 characteristic of Metaphyta and Metazoa. Between these two Volvox 

 holds an intermediate position, reminding one more of the former 

 by the possession of chlorophyll but pointing more in the direction 

 of the animal kingdom by the rest of its organisation. 



Long ago the first stage of development in Metazoa, the blastula, 

 has been compared to Volvox and was termed by Huxley') e.g. 

 "the animal Volvox". The resemblance afterwards appeared to be 

 still greater than Huxley could have suspected, for Volvox is by no 

 means a homaxone sphere rotating indiscriminate!}^ in all directions, 

 but shows a distinct opposition between an animal and a vegetative 

 pole. The line joining (hem can be described as the main axis of 

 the organism, which is not strictly globular, but a little elongated in 

 the direction of the main axis. With the animal pole directed forwards 

 it swims with a rotary movement round the main axis just as is 

 the case with the pelagic larvae of lower Metazoa and also still of 

 Amphioxus. At the animal pole the cells are smaller and further 

 from each other and contain also less chlorophyll than those at the 

 vegetative pole which are darker green, by reason of the higher 



1) BüTscHLi, 1883—1887. Protozoa II, p. 775, in Bronn's Klassen und Ord- 

 nungen des Thierreichs. 



2) T. H. Huxley, 1877. The Anatomy of Inverlebrated Animals, p. 123, 678. 



