THE DEVELOPMENT OF THE EARTH-WORM. 209 



are sufficient to give the required colour to the preparation 

 of greater or less intensity, according to desire. 



This mixture, notwithstanding its chemical irrationality, 

 gives good results. Aqueous solutions, especially when they 

 contain traces of ammonia, are to be avoided, since they are 

 very hurtful to many delicate tissues. The object must 

 remain in the dye for a period varying from a few minutes 

 to six hours, according to its size and to the nature of the 

 tissues composing it. It is a good rule, when intending to 

 make sections, to stain deeply and to cut them very thin. 



When removed from the dye the preparation is to be 

 washed in 90 per cent, alcohol, in which it may remain from 

 six to twelve hours. Finally, to remove every trace of water, 

 it should remain for half or a whole day in absolute alcohol. 



If the preparation is to be cut it must be removed from 

 absolute alcohol to essential oil of bergamot, in which it 

 should remain for some hours, in order to fit it for being 

 embedded in paraffin, which is removed from the sections 

 when cut by means of a mixture of four parts of essence of 

 turpentine with one part of kreosote. Finally, the sections 

 are mounted in resin dissolved in essence of turpentine.' 



I have made sections from the beginning of segmentation, 

 but in the earliest stages these have not been of very much 

 use, since it is impossible to place such small globular bodies 

 in a determined position ; the direction of the sections is 

 either not that required or is altogether uncertain. In conse- 

 quence I preferred studying the beginning of the development 

 by means of optical sections of the entire object, always, how- 

 ever, using real sections to control the results. 



Segmentation of the Egg and first appearance of the 

 Embryos. 



I have failed to observe the phenomena in the fecundated 

 egg immediately following the fusion of the sexual elements. 

 The earliest eggs which I have observed were already divided 

 by an equatorial furrow into two embryoplastic segments or 

 blastomeres. In this stage the egg is still contained in the 

 vitelline membrane, which is an oval capsule of about 024 

 mm. in length, whose very thin walls are without any trace of 

 structure. Its contents consist of a limpid, colourless fluid, 



^ Histologists are warned not to use a solution of resin in alcohol. 

 The preparations mounted in this are at first beautiful but soon become 

 spoiled, in consequence of the precipitation of crystals or of an amorphous 

 substance. I have lost in this manner many hundreds of preparations, 

 and the same results have occurred in the Zoological Station at Naples. 



