6 3 3 THE POPULAR SCIENCE MONTHLY. 



and form long ribbons, and that thus a large number of sections could 

 be mounted as readily as one by the former method. For some pur- 

 poses this serial section-cutting " has no especial advantages, but where 

 it is desired to preserve every section it is indispensable. To mount 

 them, however, requires special processes. The one most in use is the 

 following : 



A glass slide is coated on one side with a mixture of clove-oil and 

 collodion, which forms a viscid, sticky surface. On this the ribbons 

 of sections are laid and then the whole is exposed for some time to a 

 gentle heat in a water-bath. This melts the paraffin and allows the 

 section to drop into the sticky mixture where every part, even if sepa- 

 rate from the rest, is firmly and securely held in its proper position. 

 A continuation of the heat evaporates the clove-oil until the sections 

 are fastened to the slide by a thin film of pure collodion. The paraffin 

 is now washed away by turpentine or chloroform, Canada balsam and 

 a bit of thin glass are applied, and then every section is permanently 

 preserved and ready for study, it may be immediately or in a year 

 or two. 



Usually most of the time spent in a summer marine laboratory is 

 devoted to studying the external features of development and in pre- 

 paring material and slides of sections which can be examined more 

 carefully later. With a slide which contains all of the sections cut 

 from a single egg the student can reconstruct all the details of the 

 embryo and can trace the course and limits of every organ and vessel. 

 By comparing this with the sections of a later and an earlier stage, he 

 sees how the organs arise, and how during growth they are modified 

 in size and shape, as well as in their relations one to another. In 

 studying the living embryo, time is not taken into account. It usually 

 happens that material is obtained late in the day or in the evening. It 

 takes no rest, but develops minute by minute, hour by hour, and the 

 student, if he really wishes to master his problems must be reconciled 

 to turn night into day. His egg is changing constantly, and, with 

 pencil in hand, he sits with one eye to his microscope, watching the 

 growth. Hour after hour thus passes away, each minute revealing 

 some new feature, until at last his eyes tire and he is forced to quit. 

 He seeks his bed with regret, for he has been compelled to leave some 

 of the most interesting objects that the human eye ever saw. Consider 

 for a moment the differences between an adult fish and its minute egg, 

 clear as a drop of dew, and then try to imagine the changes which are 

 necessary to convert the one into the other. Such things as these our 

 student was forced to leave, and leave, knowing that the egg would 

 not wait for him, but on the morrow would be far advanced, and there 

 would be a great gap in his notes. While he was looking, it seemed 

 as if he could almost see actual personal life. Changes were constantly 

 occurring : now he saw a cell divide into two, just as though a knife 

 had passed through it ; again, he was seeing the way the cells thus 



