332 SUMMARY OF CURRENT RESEARCHES RELATING TO 



consequence of its appearance the ventral stolon of other Tunicates 

 has been arrested in its clevelopnaent, and has become a rudimentary 

 organ. The appearance of this new, dorsal, stolon is explained by 

 the inherited capacity of the Doliolida to produce new structures by 

 gemmation, and its supersession of the ventral one by the following 

 hypothesis: the dorsal stolon is shown to be more embryonic than 

 the ventral one by the fact of its only being formed of the three 

 germinal layers, and not, like the latter, of six rudiments ; we know 

 that embryonic tissues have a much more considerable growth-energy 

 than those that are more highly differentiated, and this advantage 

 became more and more marked by the influence of heredity. The 

 relations of the different generations is shown in the following 

 diagrams, where a letter or a combination of letters marks a genera- 

 tion, A ia a sexual, B an asexual generation, M the median, and L the 

 lateral buds. 



SynasciJiaj. Pyrosoma. Salpa. Dolioluni. 



A A 



I I 



B B. 



I I \-L 



A [A] M 



Dr. Grobben next passes to the jjhylogenetic history of alterna- 

 tion of generations in the Acalephse ; in the Hydroids, as Leuckart 

 has shown, it is due to division of labour, in consequence of which 

 only some individuals of the colony have produced generative pro- 

 ducts, and the Medusae have been derived by natural selection from 

 the free-swimming generative polyps. In the Acalephae the pheno- 

 menon is likewise due to division of labour among the members of 

 a colony. After a special reference to the studies of Professor 

 Semper, the author passes to the Cestodes, where he does not discuss 

 the question of the phylogenetic development, but merely raises the 

 question whether we have here to deal with true alternation. He 

 comes to the conclusion that it is not so, but that we have only a 

 simple metamorphosis, the larva, vesicle, scolex, and strobila being 

 one and the same individual in different stages of development. This 

 is true of the common Tcenia, but it does not apply to those cysticer- 

 coid forms in which several heads are developed, for each head 

 represents a T^enia-individual with the power of developing proglot- 

 tids. The history of the Trematoda is dealt with in the same 

 manner, and it is pointed out that we have here to do not with 

 alternation of generations, but with heterogony. The author comes 

 to the conclusion that the so-called spores are ova capable of de- 

 veloping without fertilization ; the generative products are either 

 single cells (ova), or are derived from the germinal layers of the 

 mother. In the one case we have sexual, and in the other asexual 

 development ; or, in other words, unisexual and bisexual generations 

 appear alternately in the cycle. 



