ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 979 



to four days. For preservation as museum specimens, they are killed 

 with strong nitric acid (about 50 per cent.), in which they die fully 

 extended. 



Preparation of the Ova. — The egg-capsules of fresh-water Plana- 

 rians are generally attached to water-plants by means of a white 

 secretion. The ova are very small and few in number, and are scat- 

 tered among an immense number of yolk-cells. The ova are com- 

 pletely naked, and a little smaller 4;han the yolk-cells, and are not 

 easily isolated. When cleavage begins, a large number of yolk-cells 

 surround the ovum, and form with it a mass large enough to be seen 

 with the naked eye. Jijima adopts the following mode of isola- 

 tion and preparation: — By the aid of two sharp dissecting needles, 

 the egg-capsule is opened on a slide in dilute acetic acid (2 per cent.). 

 The contents flow out, and the empty capsule is then removed. The 

 slide is next shaken, in order to isolate the ova so far as possible from 

 the yolk-cells. This process detaches many of the yolk-cells, but 

 not all ; each ovum will still have yolk-cells adhering to it, and will 

 now appear to the naked eye as a minute white mass. A cover-glass 

 supported by wax feet or by slips of paper is now placed over them. 

 After about thirty minutes the acetic acid is carefully removed by the 

 aid of small pieces of blotting-paper placed at one side of the cover, 

 and replaced by alcohol (70 per cent.). The withdrawal of the 

 acetic acid must be as slow as possible, otherwise the ova will be lost. 

 After an hour the alcohol is replaced by a stronger grade (90 per 

 cent.), in which the ova should remain two hours. Finally, the 

 alcohol is replaced by a mixture of glycerin and water in equal parts, 

 and this in turn by pure glycerin. The preparation is now complete, 

 and the cover-glass may be fixed in the usual way by means of lac. 



In order to obtain sections of embryos which are too small to be 

 treated individually, the contents of the capsule may be hardened 

 in toto in chromic acid (1 per cent.), which renders them less brittle 

 than corrosive sublimate. 



The changes which take place in the ovum initiatory to cleavage 

 are very difiicult to trace, as they are generally completed before the 

 cocoon is laid. In some cases ova were found in fresh laid capsules, 

 which showed the germinal vesicle still unchanged ; others were 

 found to have two nuclei, supposed to be derivatives from the first 

 cleavage nucleus. This stage of two nuclei was also found in some 

 cocoons taken directly from the penial sheath, in which the cocoon 

 formation takes place. It is therefore not quite certain when fecun- 

 dation takes place, whether in the cocoon or before its formation. 



Starch Injection Mass.* — Prof. S. H. Gage considers that a coarse 

 injection mass which is cold- flowing, which may be forced nearly to 

 the capillaries, rapidly hardening after injection and leaving the 

 vessels flexible, which docs not dull dissecting instruments, and is 

 suitable for permanent dry or alcoholic preparations, being at the same 

 time simjde in its manipulation, cleanly and economical, is fully 



♦ Amer. Natural., xviii. (1884) pp. 958-60, from 'New York MedicalJournal,' 

 7th June, 1884. 



