768 SUMMARY OF CURRENT RESEARCHES RELATING TO 



it contains spheroidal refringent myeloid globules, coloured black by 

 osmic acid ; they multiply rapidly, but remain distinct and are com- 

 parable to the myelin of Vertebrata. Pigment-granules of a brownish 

 or yellowish colour, found in some ganglia, are often seen in the pro- 

 toplasm of the nerves ; they are not altered by ether or chloroform. 

 Transverse sections of the nerves show this protoplasmic layer to 

 constitute the entire covering of the central fibrils, and to be approxi- 

 mately homogeneous in density throughout ; although a slightly 

 denser external zone is somewhat constant in its occurrence, it is not 

 dense enough to be comparable to the Schwann's sheath of the Verte- 

 brate nerve. 



Molluscoida. 



Disdaplia* — A. Delia Valle, in describing this new genus of the 

 Synascidise, states that the tail of the larva presents the following 

 constitution : there is an envelope of cellulose, with amoeboid nuclei, 

 a membrane continuous with the ectoderm, which is formed of large, 

 flattened epithelial cells, a contractile layer of fusiform cells which 

 are transversely striated, and the axis of the tail, which is more 

 transparent than the rest, and is occupied by the hyaline cylinder, 

 which is. according to some, a solid cartilaginous notochord ; the 

 author, however, like some other writers, finds that this axial struc- 

 ture is a hollow tube, the wall of which is continuous with that of the 

 peritoneal sac. 



Attention is to be directed to the fact that the first buds which are 

 developed in a young colony have no sexual glands, but those that are 

 derived from the later colonies (due themselves to the repeated fission 

 of the primitive buds) present at once indications of these glands, or 

 at least of groups of cells which will develope into them. The author 

 promises to prove in a later work that this phenomenon is not peculiar 

 to Disdaplia, but is to be seen in other Synascidians. 



Natural History of Doliolum.t — B. Uljanin concludes that, in 

 the developmental cycle of Doliolum, only two generations succeed one 

 another : one of them is developed from the egg, is provided with a 

 stolo prolifer, and gives rise to a generation of nurses ; the other is 

 derived asexually from the stolon, and this latter generation is poly- 

 morphous ; the separate forms which constitute it have hitherto been 

 regarded as special generations, and distinguished as lateral buds, 

 median buds (second generation of nurses), and sexual forms ; of 

 these, the two former have rudiments of reproductive Organs, which 

 disappear during the course of development. Doliolum may be 

 looked upon as a form which has inherited a process of alternation of 

 generations from the Synascidia? and Pyrosomidce, and in which, 

 owing to a slow diminution in the amount of nourishment, the nurse 

 developed from the ovum has gradually been subjected to a series of 

 adaptation, for the purpose of the preservation of the species. The 

 want of true colonial life has diminished the amount of gemmation 



* Arch. Ital. de Biol., i. (1882) pp. 193-203. 

 t Zool. Anzeig., v. (1882) pp. 429-36, 447-53. 



