PREFACE. 



It is the object of this work to place before teachers and students of biology a series of figures, photographed directly 

 from nature, to illustrate some of the principal phenomena in the fertilization and early development of the animal ovum. 

 In no branch of biological incjuiry has knowledge advanced of late with such rapid strides as in the new science of 

 cytology, which deals with the internal phenomena of cell-life. Within the past two decades this science has brought 

 forward discoveries relating to the fertilization of the egg and the closely related subjects of cell-division and karyokinesis 

 that have called forth, on the part of Weismann and others, some of the most important and suggestive discussions of the 

 post-Darwinian biology. These discoveries must in some measure be dealt with by every modern text-book of morphology 

 or physiology, yet they belong to a region of observation inaccessible to the general reader or student, since it can only be 

 approached by means of a refined histological technique applied to special objects not ordinarily available for practical 

 study or demonstration. A knowledge of the subject must, therefore, as a rule, be accjuired from text-books in which 

 drawings are made to take the place of the real object. 



But no drawing, however excellent, can convey an accurate mental picture of the real object. It is extremely difficult 

 for even the most skilful draughtsman to represent the exact appearance of protoplasm and of the delicate and 

 complicated apparatus of the cell. The best drawing must necessarily be in some measure schematic and embodies a 

 considerable subjective element of interpretation ; it is, moreover, impossible adecjuately to reproduce it in a black and 

 white text-book figure. The photograph, whatever be its shortcomings (and no photograph can do full justice to nature), 

 at least gives an absolutely unbiassed representation of what appears under the microscope; it contains no subjective 

 element, save that involved in focussing the instrument, and hence conveys a true mental picture. Even a technically 

 perfect photograph, however, is defective in that it sharply reproduces only what is seen at a single level of the focus. 

 In using high powers, moreover, the sharp image at the exact focus is always blurred to some extent by indistinct images 

 of higher and lower levels, and this is the case with even the thinnest sections. Protoplasm is thus made to appear in 

 the photograph more coarsely granular than it does to the eye, the asters are less sharp and brilliant, the apparent size of 

 chromosomes and other minute bodies may be slightly exaggerated, etc. Nevertheless, on the whole, these unavoidable 

 defects of the photograph introduce negative rather than positive errors, — they are faults of omission rather than 

 commission, — and I believe that the photographic plates here presented give, on the whole, a clear and accurate impres- 

 sion of the preparations. 



How far, then, do the preparations themselves correspond with the conditions existing in life, and what sources of 

 error may be sought in the methods of preservation .-* This cjuestion is always a difficult one to answer in work of this 

 character, since, as a rule, many of the most important elements of cell-structure are invisible in life, and can only be 

 brought to view by means of suitable fixation, staining, and clearing. In the present case, however, the eggs are so 

 transparent as to show many of the phenomena (though by no means all) in life so that the preparations could be 

 carefully tested by direct comparison with the living object. The result of such tests, which have been very carefully 

 made, shows that the methods employed afford preparations whose general fidelity to nature is beyond question, and I 

 believe, for reasons stated below, that they may be trusted even for the minutest details. 



