CHROMATOLYSIS. 



77 



After opening the abdominal wall of the animal, the ovisacs are 

 removed, their numerous convolutions separated as much as possible, 

 and then fixed for twenty-four hours in a picric-acetic acid solution 

 (a concentrated aqueous solution of picric acid diluted with 2 vols. 

 of water to which i per cent, glacial acetic acid is added). Then fol- 

 lows washing for twenty-four hours with water, after which the specimen 

 is transferred to increasing strengths of alcohol (Boveri, ibid,^). Differ- 

 ent regions of the ovisacs contain ova in various stages of development, 

 those nearest the head containing cells ripe and ready for fecundation, 

 while in the more posterior regions are ova in varying stages of segmen- 

 tation showing mitoses. Specimens fixed in the manner above described 

 can be stained with a borax-carmin solution. After staining, the ova are 

 gently pressed out with needles upon a slide, separated, covered with a 

 cover-glass, and cleared by gradual irrigation with glycerin. The ova, 

 especially the segmentation spheres, are very small, and can be examined 

 only under high magnification. In spite of the minuteness of the ob- 

 ject and the fact that the yolk does not take the stain, and, on account of 



v: v ; ;!vll_Centrosphere. 



Nucleus. 



Fig. 37. Pigment cell from the skin of the head of a pike ; X 650. 



its high refractive index, distorts the picture to a considerable extent, the 

 mitotic figures are beautifully distinct. 



Certain methods of treatment bring out in both cells and nuclei 

 the presence of peculiar granules. The latter have been especially 

 studied and described by v. Altmann (94, 2ded. ). The methods that 

 he applies are as follows : The specimens of organs of recently killed 

 animals are fixed in a mixture consisting of equal volumes of a 5% 

 aqueous solution of potassium bichromate and a 2 fc solution of osmic 

 acid, remaining in the mixture for twenty-four hours. They are then 

 washed for several hours in water and treated with ascending strengths 

 of alcohol ; viz., 70, 90, and 100%. The specimens are now placed in 

 a solution of 3 parts of xylol and i part of absolute alcohol, then in 



