ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 51 



(mechanical) conditions which have inhibited the distinct formation of 

 the primary germinal layers. 



Position of Ligula.* — Dr. 0. von Linstow gives a diagnosis of the 

 genus : — In the dorsal and ventral median line a longitudinal groove ; 

 rudimentary dorso-ventral suckers without special musculature ; no 

 formation of proglottides, " segmentation " absent in the larva and 

 restricted to the anterior third of the body in the adult ; gonads as in 

 Bothrioceplialus, but compressed into closely compacted successive 

 groups to which the segmentation does not correspond ; cirrus, vagina, 

 and uterus open beside o)ie another in the ventral median line in a 

 genital sinus in a transverse row (the vagina in the middle, the cirrus 

 sometimes right, sometimes left) ; excretory vessels in the larva in the 

 medullary and cortical layers, in the sexual form two sets of longitudinal 

 vessels in the cortical layer. 



Therefore, he says, the genus Ligula belongs to the Bothrioeephalidas 

 along with the nearly related Scliistocephalus. 



Life-history of Distomum folium.f — D. Th. Ssinitzin has found that 

 the first host of this fish-parasite is Dreissensia polymorpha. The mira- 

 cidium probably enters with the water of respiration ; it at least finds its 

 way to the gills, there loses its covering of cilia, and by peristaltic 

 contractions of the body forces its way into the interfoliar space of the 

 gills. There it becomes a sporocyst and within the body germ-cells 

 develop, and produce 12-14 new sporocysts. The process is repeated 

 several times, until the interfoliar spaces of the host's gills become 

 crowded with the parasites. The last generation includes forms of large 

 size, with the cells of the body-wall crowded with drops of fat, and 

 containing cercarige in various stages of development. Absolute proof 

 of the identity of these with _D. folium was obtained by removing speci- 

 mens from the sporocysts, and rearing them in a nutritive solution. 

 Successful infection experiments were also made with fish in aquaria. 

 Full details are promised later. 



Early Stages in Development of Polystomum integerrimum.l — 

 Dr. H. Halkin finds that there is the usual period of maturation resulting 

 in the elimination of two polar bodies ; that these, though very small, 

 are formed from a karyokinetic figure occupying the whole diameter of 

 the ovum ; that the first maturation division is characterised by the 

 presence of central corpuscles in the form of long bent rods (divided or 

 not), while the second division shows none ; that the pronuclei, lobulated 

 from the start, persist throughout, enclosing the nucleoli from their 

 formation, and subsequently exhibiting a chromatic network; that the 

 first division figures show large central corpuscles, spherical and slightly 

 stainable ; that the typical number of chromosomes is twenty ; and that 

 the aspect of the vitellus is characteristic for each of the three successive 

 mitotic figures which are formed in it. 



The cleavage is unequal and " adiaphorogenetic " — a term used by 

 Hallez to express the fact that the blastomeres are not from the first 

 specifically differentiated (e.g. into ectoderm and endoderm). It may 

 be permitted to us to hope that the ungainly term will have a short life. 



* Zool. Anzeig.. xxiv. (1901) pp. 627-34 (1 fig.). f Tom. cit., pp. G89-94. 

 X Arch. Biol., xviii. (1901) pp. 291-363 (5 pis.). 



E 2 



