4 J. GEAHAM KERR. 



them were now laid in a bath of absolute alcohol, to remove 

 the cedar oil, and then taken up and laid sections downwards 

 upon slides coated with a layer of dry collodion. A finger 

 was now passed lightly along the paper, giving a gentle 

 pressure, just sufficient to cause the celloidin of the sections 

 to adhere to the collodion on the slide. The slide was now 

 removed to 90 per cent, alcohol and the ordinary process of 

 staining carried out. In the subsequent dehydration pre- 

 vious to mounting a mixture of chloroform and absolute 

 alcohol was used for the final stage of the process. 



II. To obtain thin sections of yolk-laden eggs, it was 

 necessary to embed in both celloidin and paraffin. The pre- 

 liminary embedding in celloidin was done as before. The 

 egg was taken from celloidin solution and dropped bodily 

 into chloroform for 15 — 30 minutes. At first I was in the 

 habit of transferring the celloidin block to cedar oil before 

 embedding in paraffin, but latterly I have frequently em- 

 bedded at once by the chloroform-parafiin method. It is of 

 great importance to keep the temperature of the water-bath 

 as low as possible, and also to diminish the length of time 

 during which the object is on the water-bath to the shortest 

 possible. 



Sections were cut with a Cambridge rocking microtome, 

 and flattened with warm water on a slide coated with 

 glycerine and egg albumen. The water was drained off 

 and the slides put aside to dry in an atmosphere containing 

 vapour of alcohol and ether. It was found that drying in 

 an ordinary atmosphere over the water-bath caused the 

 celloidin-infiltrated section to di-y, curl up, and break away 

 from the paraffin : this was avoided by drying in the manner 

 described. It is important, however, not to use an atmo- 

 sphere completely saturated with ether and alcohol vapour, 

 as this, by causing the celloidin to swell, may cause wrinkling 

 of the sections. 



III. Older embryos were embedded in paraffin in the 

 ordinary way and cut with the rocking microtome. 



Orientation. — For the accurate orientation of embryos 



