500 A. M. CAllH SAUNDERS AND MARGARET FOOLE. 



far the best of all tlie fixing- agents which were tried, though 

 for special purposes others were iised, as, for example, 

 Hermann's fluid to show up the liver. 



AV^hen a thread of spawn was laid it was taken and sus- 

 pended in the tank or jar by means of a string. When the 

 embryos had reached a stage which it was desirable to 

 preserve, a piece an inch or so in length was cut off the end 

 of the thread, divided into a number of fragments a few 

 millimetres long, and put in the picric and forinol solution for 

 about half an hour. At the end of that time it was easv to 

 break the capsules with a needle ami extract the embryos, 

 the greater number of them entirely uninjured. Various 

 stains were used, but paracarmine gave the best results for 

 whole preparations ; sections were stained on the slide witli 

 borax-carmine, followed by picro-indigo-carmine. 



The eggs of Aplysia are small, being less than 100 /a in 

 diameter, and this makes orientation before section-cuttiny: 

 pi-actically impossible. In the end, therefore, it was found 

 more convenient to embed large numbers close together which 

 could all be cut at the same time, for in this way one could 

 be certain of getting a few embryos cut in the plane that was 

 desired. In order to embed a laro-e number of eq-gs in a 

 small area of paraffin the following method was employed — a 

 watchglass was filled Avith paraffin and allowed to cool ; a 

 small round hole, reaching at least half way through the 

 ]iaraffin, was then made ; the embryos were transferred into 

 this by means of a fine pipette, and as much xylol as possible 

 drawn off. The watch glass was then placed for half an hour 

 on a stand on the warm bath, for half an hour on the bath 

 itself, and finally inside the bath until the paraffin melted 

 completely, when it was cooled. 



More points remain undecided in the ontogeny of Molluscs 

 than perhaps m any other group in the Animal Kingdom. 

 The cell lineao-e has been worked out in numerous cases 



O 



among the various groups, but our knowledge of those stages 

 which follow upon the end of segmentation is very incomplete. 



