HUMAN EMBRYOLOGY. 



By A. H. YOUNG and ARTHUR ROBINSON. 

 Rewritten by ARTHUR ROBINSON. 



THE ontogenetic or developmental history of every human individual is separable 

 into two main periods, pre-natal and post-natal. 



It is to the knowledge of the phenomena of the earlier or pre-natal period that 

 the term human embryology is applied, and as pre-natal development takes place in 

 an organ called the uterus, it is frequently spoken of as intra-uterine development. 



The period of pre-natal development extends through nine lunar months, and 

 may be divided into three sub-periods : (1) the pre-embryonic period, during which 

 the zygote, from which the embryo is formed, shows no definite separation into 

 embryonic and non-embryonic portions. This period lasts about fourteen days ; 

 (2) the period of the embryo, in which the zygote is definitely separated into 

 embryonic and non-embryonic portions, but the embryonic part has not yet 

 assumed a clearly human form. This period terminates at the end of the second 

 month ; (3) the foetal period, which commences at the end of the second month, 

 when the embryo assumes a definitely human form and is called, thenceforth, a 

 foetus. The fetal period ends at birth, when the foetus becomes a child and post- 

 natal development commences. 



Only the general phenomena of the pre-natal period of development are 

 considered in this section; the details of the pre- and post-natal development 

 of the various organs and systems will be dealt with in the sections devoted to 

 the descriptions of their adult conditions. 



THE STRUCTURE OF ANIMAL CELLS. 



The human body is formed by the multiplication and differentiation of animal 

 cells, therefore it is essential that the student should possess a knowledge of the 

 main features and capabilities of 

 such cells before he commences 

 the study of the details of human 

 embryology. 



Nucleus 



Animal cells differ from each 

 other in minor points of structure, 

 in association with the positions 

 they occupy and the functions they 

 perform; nevertheless, they all 

 possess some common and essential 

 structural features, and, in the 

 younger stages . of their history, 

 some common capabilities. 



The following are the constituent parts of a typical animal cell : 



The cell body : containing 



(a) The nucleus with its nucleolus ; 



(&) The centrosome with the centrioles ; 



(c) The mitochondria. 



All the essential parts of the cell consist of a substance called protoplasm. 

 In its simplest form protoplasm is the semifluid, viscous, irritable, and con- 



'7 



Nucleolus 



Nuclear 



Spongioplasm 

 (cyto-reticulum) 



- HER Hyaloplasm 



Attraction sphere 



Centrosome 



FIG. 2. DIAGRAM OF AN ANIMAL CELL. 



