INVERTEBBA.TA, CRYPTOGAMIA, MICROSCOPY, ETC. 415 



the * MorpLol. Jahrbuch,' ii. p. 440 ; here their rotation soon becomes 

 slackened, and that long before any change occurs in the form or 

 structure of the eggs. With regard to the method of observation by 

 sections, it is necessary to point out that these are only really useful 

 where every section is carefully numbered, and no part is lost at all. 

 The sections should be strictly parallel, and have a definite direction ; 

 of this last there should, in each important case, be three ; one trans- 

 verse, one "facial," and one sagittal. The longitudinal axis is that which 

 passes through the mouth and the aboral axis of the body; the transverse 

 axis is perpendicular to this ; the sagittal plane passes through the longi- 

 tudinal axis and the median ventral and dorsal lines, and all sections 

 parallel to this jjlane are sagittal sections. The facial jjlane likewise 

 passes through the longitudinal axis, but it is perpendicular to the 

 two preceding ones, and is parallel to the dorsal and ventral faces of 

 the embryo. The embryos are isolated by plunging the eggs in very 

 dilute chromic acid ; in this the embryos harden, while the albumen 

 remains comparatively soft and can be easily removed, and the 

 embryos can then be hardened in successive quantities of alcohol ; to 

 remove the shells, where present, it is only necessary to add a few 

 drops of nitric acid to the alcohol. 



Development of Lymnaeus stagnalis.* — Herr Wolfson's method 

 of examination was to harden the ova in one-third per cent, chromic 

 acid, wash in water, place in various solutions of alcohol, embed in 

 paraffin, and cut by Leiser's microtome ; the sections were stained 

 with picrocarmine, clarified in lavender oil, and put up in Canada 

 balsam. Eggs which had been hardened in chromic acid and then 

 treated with even boiling water, also gave very good results. 



As a result of the early stages, the author finds that the whole 

 amphiaster is a nuclear, but not a protoplasmic structure, and that the 

 rays of the stellate figure are not expressions of any radiate arrange- 

 ment of the protoplasm, but a kind of pseudopodia ; the so-called 

 spindle-nucleus is nothing more than the point of interlacement of the 

 rays of the two stars. He finds, moreover, nothing to sui)port the 

 doctrine of attraction centres. 



In the early processes of cleavage four stages are to be distin- 

 guished ; tlie first is a j^assive one, and is characterized by the feeble 

 power of attraction possessed by the cleavage spheres ; the second is 

 sevii-jiassive, tlie cleavage spheres run together, but the nuclei are still 

 colourless and vacuolar; the third stage is active, for tlie cleavage- 

 groove is obscured by the attraction of the splieres, the nuclei become 

 intensely coloured, and take on their definite form. In the fourtli or 

 sevn-odivc stage, which is always preliminary to a new division, the 

 8i)hcre8 6ei)aratc from one another. 



After shortly describing the morula starje, the author says that he 

 has given very great care to the investigation of tlie gastrula, and ho 

 finds tliat in Lymnaus a gastrula without a mesoderm does not exist, 

 and, moreover, cannot exist, inasmuch as the mesodermal cells arc 

 formed in an early cleavage stage. 



• ' Bull. Aciul. Imp. St. Petersbourp,' xxvi. (1880) p. 7'.>. 



