266 HUMAN PHYSIOLOGY. 



Fecundation is the union of the spermatozoa with the ovum during 

 its passage toward the uterus, and usually takes place in the Fal- 

 lopian tube, just outside the womb. After floating around the ovum 

 in an active manner, they penetrate the vitelline membrane, pass into 

 the interior of the vitellus, where they lose their vitality, and, along 

 with the germinal vesicle, entirely disappear. 



DEVELOPMENT OF ACCESSORY STRUCTURES. 



Segmentation of the Vitellus. After the disappearance of the 

 spermatozoa and the germinal vesicle there remains a transparent, 

 granular, albuminous substance, in the center of which a new nucleus 

 soon appears ; this constitutes the parent cells, and is the first stage 

 in the development of the new being. 



Following this, the vitellus undergoes segmentation ; a constric- 

 tion appears on the opposite side of the vitellus, which gradually 

 deepens, until the yolk is divided into two segments, each of which 

 has a distinct nucleus and nucleolus ; these two segments undergo 

 a further division into four, the four into eight, the eight into 

 others, and so on, until the entire vitellus is divided into a great 

 number of cells, each of which contains a nucleus and a nucleolus. 



The peripheral cells of this " mulberry mass " then arrange them- 

 selves so as to form a membrane, and, as they are subjected to mutual 

 pressure, assume a polyhedral shape, which gives to the membrane 

 a mosaic appearance. The central part of the vitellus becomes 

 filled with a clear fluid. A second membrane shortly appears within 

 the first, and the two together constitute the external and internal 

 blastodermic membranes. 



Blastodermic Membranes. The embryo, at this period, consists 

 of three layers viz., the external and the internal blastodermic 

 membranes and a middle membrane formed by a genesis of cells 

 from their internal surfaces. These layers are known as the epi- 

 blast, mesoblast, and hypoblast. 



The epiblast gives rise to the central nervous system, the epidermis 

 of the skin and its appendages, and the primitive kidneys. 



The mesoblast gives rise to the dermis, muscles, bones, nerves, 

 blood-vessels, sympathetic nervous system, connective tissue, the 

 urinary and reproductive apparatus, and the walls of the alimentary 

 canal. 



