xiv INTRODUCTION. 



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

 the first accurate differentiation between the Graafian follicle and the ovurr,. 

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

 organic structure. Only comparatively gross Embryology was thus possible 

 With the recognition of the cell as the basis of animal structure (Schleiden an: 

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

 the ovum became known as a typical cell, while a little later (Kolliker, Reichei 

 and others, about 1840) was established the function of the spermatozoo; 

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

 may consider the two fundamental facts of Histology and of Embryology 

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

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

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

 cell the fertilized ovum. 



The adult body being thus composed of an enormous number of cells, var) 

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

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

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

 and. female germ cells to the cessation of developmental life. 



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

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

 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 structur 

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

 proliferation. 



While the subject of this work is distinctly human Embryology, it is neithe 

 possible nor advisable to confine our study wholly to human material. It is no 

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

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

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

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

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

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

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

 satisfactory if the student has sufficient knowledge of comparative anatomy, an< 

 the deductions regarding human development, from the study of developmen 

 ia lower forms, are rarely in error. 



