GENERAL EMBRYOLOGY. 
By Atrrep H. YounG and ARTHUR ROBINSON. 
ALTHOUGH the tissues and organs of the body when fully formed differ greatly 
not only in respect of their functional characteristics but also with regard to their 
structural features, they are developed from cell elements which, at first, cannot be 
distinguished from one another, and all of which are the offspring of parent cells— 
the female cell or ovum, and the male cell or spermatozoon. Developmental processes 
take place in the female cell alone, but they cannot occur unless the essential 
elements of a sperm or male cell previously unite with it. 
Like all animal cells, the ovum is a mass of protoplasm (cytoplasm) containing 
a nucleus. In many cells the cytoplasm or cell body is itself enclosed by an ex- 
ternal investing membrane, the cell wall, and probably there is such a membrane 
in the ovum. Speaking generally, animal cells are minute structures, those of the 
human body rarely attaining a diameter of more than about °083 mm., but they 
vary somewhat in size, they assume different forms, and they acquire characteristic 
peculiarities associated with their positions and functions; thus, whilst the majority 
of the constituent cells of an individual form the various tissues and organs of the 
body, others become reproductive or germinal cells. 
Ova are simply specialised cells modified and adapted for the purpose of repro- 
duction and the continuance of the species. They are developed in the ovary, 
one of the female generative organs, in the cell-lined spaces known as Graafian 
follicles. 
When an ovum has reached a certain stage of development it is discharged from 
the ovary, and passing along the oviduct or Fallopian tube it eventually reaches 
the cavity of the uterus. Though mature and capable of being fertilised it may 
not be impregnated, in which case it does not remain in the uterus but is cast out 
from that organ. If, however, it becomes fertilised by union with the male germinal 
element it is retained in the uterus, and develops {into an embryo which possesses 
all the characteristic features of the species to which it belongs and most of the 
special peculiarities of its parents. 
When the embryo, or the foetus as it is termed after it has assumed definite 
form, is capable of independent existence, its intrauterine life terminates, and 
it is separated from the rest of the ovum and is born. The development of the 
individual, however, is not complete, nor does it become complete until the new 
being reaches the adult condition. 
The term embryology is sometimes used to include the consideration of all the 
developmental changes and processes which take place in the ovum from the begin- 
ning up to the final adult stage. It is more convenient, however, to restrict its 
application to the study of those changes which take place during the development 
and growth of the organism before the fcetus is separated from the ovum, or, in 
other words, during its intrauterine existence. 
Briefly epitomised, the sequence of changes is as follows. Impregnation of the 
primitive nucleated ovum is followed by segmentation or cleavage. By a series of 
successive divisions the egg-cell is divided into two, four, eight, and ultimately into 
a large number of cells, and so is transformed into a multicellular mass, the morula, 
