EMBBYOLOGY. 



SECTION I. 



GENEKAL EMBKYOLOGY. 



DEVELOPMENT in the human being, as in all the Metazoa, is initiated by the union 

 of two specialised cells the germ-cell or ovum, and the sperm-cell or spermato- 

 zoon. The ovum, after its union with the spermatozoon, may be conceived as 

 the central point of a developmental cycle. By its continuous division it gives rise 

 to the multicellular body, or soma. At a certain stage, in some animals at a very 

 early stage, of development, there is isolated from the mass of somatic elements 

 a stirp of cells destined for the reproduction of the species. These are located 

 in the reproductive glands the testis in the male and the ovary in the female 

 and there undergo an elaborate series of changes which result in the production of 

 the mature sex-cells, by the union of which a new cycle is again initiated. 



From this point of view we may divide the history of development into two 

 sections : 



A. The history of the soma. 



B. The history of the sex-cells. 



In describing the course of development, it is most convenient to begin at a 

 point in the cycle at which the reproductive stirp is already laid down, and the 

 phase has been entered on which leads to the specialisation of the sex-cells. 



In order to apprehend clearly the nature and significance of the process of 

 specialisation of the sex-cells, as well as the general processes of histogenesis of the 

 somatic elements, it is necessary that the reader should have some knowledge of 

 the structure of the cell, as the primal element out of which the adult organism is 

 developed, and the morphological unit of all the tissues and organs of which it is 

 composed. 



THE ANIMAL CELL. 



The animal cell is a minute body of microscopic dimensions, consisting 

 of a speck of living substance of semi-fluid consistence and complex chemical 

 composition, known as protoplasm. It may or may not possess a limiting 

 membrane differentiated from the surface-layer of the protoplasm, but always 

 contains a minute vesicular body within it named the nucleus. 



Cytoplasm. The protoplasm of the cell-body is known as the cytoplasm, 

 to distinguish it from that of the nucleus, which is termed karyoplasm. A 

 complete and critical account of the physical characters and composition of the 

 cytoplasm will be found in the volume of this work devoted to general histology ; 

 here it will be necessary only to refer to some of the more important points. In 

 the living condition, it has a homogeneous glassy appearance, with or without 

 imbedded granules, and a semi-fluid consistency. In some cases it shows, 

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