8 GENERAL EMBRYOLOGY. 
The majority of the “segmentation masses” or cells, blastomeres, as they are 
termed, are differentiated into tissue elements, but a certain number retain the 
characters of the original germ-cells and become ova or sperm-cells, which form the 
“points of departure ” of succeeding generations. Every germ- cell is derived, 
therefore, “ by a continuous and unbroken series of cell divisions” which have ex- 
tended through the past, from the most primitive ancestor, and it forms a point 
from which, under ordinary circumstances, all future generations will commence. 
It is in this sense that the changes through which a living being passes in the 
course of its life “may, in their completest form, be considered as constituting 
a morphological cycle, beginning with the ovum and ending with the ovum 
again.” 
To follow these changes it is necessary that the characters and capabilities of 
the constructive elements should be clearly understood. The animal cell, which 
plays an all-important part in the life-history of the individual, and the modified 
germ-cells must be carefully studied, and as far as possible the exact nature of their 
constituent parts ascertained. 
The phenomena of impregnation and segmentation, and the subsequent develop- 
mental processes and morphological changes which result in the formation of the 
embryo, and, finally, the arrangements for the nutrition and protection of the ovum 
during its intrauterine existence, will then be considered. 
THE ANIMAL CELL. 
Cells are the structural units of the body. Each cell has an individual life- 
history within the tissue or organ to which it belongs, it is produced by a pre- 
existing cell, it develops and grows, is modified by circumstances, reproduces other 
cells similar to itself, and dies. 
A cell possesses a body and a nucleus. An external investing membrane 
or cell wall may or may not be differentiated. 
The cell body consists of proto- 
Spongioplasm plasm—an unstable, highly complex 
Nucleolus (cyto-reticulum) on oe G : 
organic substance, the constitution 
Hyaloplasm of which is approximately repre- 
IOI sented by the formula CoH o9N jgOap. 
It is colourless, semi-fluid, viscous, 
insoluble in water, capable of 
osmosis, and it is contractile and 
irritable. In the living condition 
it always contains a certain amount 
of water and various inorganic 
matters. It is to be observed, how- 
ever, that there are many varieties 
of protoplasm, differing somewhat in nature and qualities. 
The protoplasm of the cell body is.called cytoplasm. Under low powers of the 
microscope it is homogeneous or slightly granular, but with higher magnification, 
and especially after the application of staining agents, it is possible to distinguish— 
(1) A highly refractile, elastic, and extensile network—the cyto-reticulum or 
spongioplasm—the meshes of which are filled with 
(2) A clear, semi-fluid substance—the cytolymph or hyaloplasm. 
The fibres of the reticulum present some few minute rounded bodies of doubtful 
nature, which are termed microsomes. 
The nucleus is a spherical vesicle embedded in the cell body. It is surrounded 
by a distinct nuclear membrane, and usually contains nucleoli. 
It consists of modified protoplasm, which is termed karyoplasm, the precise rela- 
tion of which to the cytoplasm is not clear. Structurally it resembles cytoplasm in 
that it presents a fine reticulum, the fibres of which seem to be continuous with the 
cyto-reticulum through the nuclear membrane, whilst its meshes are occupied by 
nuclear juice. 
The reticulum forms a fine network composed of linin fibres (achromatic 
— Attraction sphere 
Nuclear 
membrane 
= 
Centrosome 
Fic. 2,—DIAGRAM OF AN ANIMAL CELL. 
