xvi INTRODUCTION. 



in it we have the first definite description of the primary germ layers as well as 

 the first accurate differentiation between the Graafian follicle and the ovum. 

 It will be remembered that the cell was not as yet recognized as the unit of 

 organic structure. Only comparatively gross Embryology was thus possible. 

 With the recognition of the cell as the basis of animal structure (Schleiden and 

 Schwann, 1839) the entire field of histogenesis was opened to the embryologist ; 

 the ovurn became known as a typical cell, while a little later (Kolliker, Reichert 

 and others, about 1840) was established the function of the spermatozoon 

 and the fact that it also was a modified cell structure. From this time we 

 may consider the two fundamental facts of Histology and of Embryology, 

 respectively, as firmly fixed beyond controversy; for Histology, the fact that 

 the body consists wholly of cells and cell derivatives; for Embryology, the 

 fact that all of these cells and cell derivatives develop from a single original 

 cell the fertilized ov.um. 



The adult body being thus composed of an enormous number of cells, vary- 

 ing in structure and in function, forming the different tissues and organs, and 

 these cells having all developed from the single fertilized germ cell, it is the 

 province of Embryology to trace this development from the union of male 

 and female germ cells to the cessation of developmental life. 



While Embryology thus properly begins with the fertilized ovum, that is, 

 with the first cell of the new individual, certain preliminary considerations are 

 essential to the proper understanding of this cell and its future development. 

 These are the structure of the ovum and of the spermatozoon and their de- 

 velopment preparatory to union. Also, as it is with cells and cell activities 

 that Embryology has largely to deal, it is necessary to consider the structure 

 of the typical animal cell and the processes by which cells undergo division or 

 proliferation. 



While the subject of this work is distinctly human Embryology, it is neither 

 possible nor advisable to confine our study wholly to human material. It is not 

 possible, for the reason that material for the study of the earliest stages in the 

 human embryo (first 12 days) is entirely wanting, while human embryos of 

 under 20 days are extremely rare. Again, even later stages in human develop- 

 ment are often best understood by comparison with similar stages in lower 

 forms. For practical study by the student, human material for all even of 

 the later stages is rarely available, so that recourse must frequently be had to 

 material from lower animals. Such study is, however, usually thoroughly 

 satisfactory if the student has sufficient knowledge of comparative anatomy, and 

 the deductions regarding human development, from the study of development 

 in lower forms, are rarely in error. 



