EMBRYOLOGY. 





THE term Embryology, in its widest sense, is applied to the various changes 

 which take place during the growth of an animal from the egg to the adult 

 condition: it is, however, usually restricted to the phenomena which occur before 

 birth. Embryology may be studied from two aspects: (1) that of ontogeny, which 

 deals only with the development of the individual; and (2) that of phytogeny, 

 which concerns itself with the evolutionary history of the animal kingdom. 



In vertebrate animals the development of a new being can only take place when 

 a female germ cell or ovum has been fertilized by a male germ cell or spermatozoon. 

 The ovum is a nucleated cell, and all the complicated changes by which the various 

 tissues and organs of the body are formed from it, after it has been fertilized, are 

 the result of two general processes, viz., segmentation and differentiation of cells. 

 Thus, the fertilized ovum undergoes repeated segmentation into a number of cells 

 which at first closely resemble one another, but are, sooner or later, differentiated 

 into tw T o groups: (1) somatic cells, the function of which is to build up the various 

 tissues of the body; and (2) germinal cells, which become imbedded in the sexual 

 glands the ovaries in the female and the testes in the male and are destined for 

 the perpetuation of the species. 



Having regard to the main purpose of this work, it is impossible, in the space 

 available in this section, to describe fully, or illustrate adequately, all the phenom- 

 ena which occur in the different stages of the development of the human body. 

 Only the principal facts are given, and the student is referred for further details 

 to one or other of the text-books 1 on human embryology. 



THE ANIMAL CELL. 



All the tissues and organs of the body originate from a microscopic structure 

 (the fertilized ovum), which consists of a soft jelly-like material enclosed in a 

 membrane and containing a vesicle or small spherical body inside which are one 

 or more denser spots. This may be regarded as a complete cell. All the solid 

 tissues consist largely of cells essentially similar to it in nature but differing in 

 external form. 



In the higher organisms a cell may be defined as "a nucleated mass of proto- 

 plasm of microscopic size." Its two essentials, therefore, are: a soft jelly-like 

 material, similar to that found in the ovum, and usually styled cytoplasm, and a 

 small spherical body imbedded in it, and termed a nucleus. Some of the unicellular 

 protozoa contain no nuclei but granular particles which, like true nuclei, stain with 

 basic dyes. The other constituents of the ovum, viz., its limiting membrane and 

 the denser spot contained in the nucleus, called the nucleolus, are not essential ta 

 the type cell, and in fact many cells exist without them. 



Cytoplasm (protoplasm) is a material probably of variable constitution during 

 life, but yielding on its disintegration bodies chiefly of proteid nature. Lecithin 

 and cholesterin are constantly found in it, as well as inorganic salts, chief among 



1 Manual of Human Embryology, Keibel and Mall; Handbuch der vergleichenden und experimentellen Entwickel- 

 ungslehre der Wirbeltiere, Oskar Hertwig; Lehrbuch der Entwickelungsgeschichte, Bonnet; The Physiology of 

 Reproduction, Marshall. 



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