THE PRINCIPLES OF EMBRYOLOGY 481 
utilized either as yolk-cells for the providing of pabulum to the eyg- 
cell, or as excretory cells for the removal and rendering harmless of 
deleterious products of all kinds. Thus the free cells of the body 
would become differentiated into the three classes of germ-cells, 
yolk-cells, and excretory cells. 
Further, the mass of gonads, which originally occupied so large 
a space within the interior of the host, necessarily, as the tissues of 
the host differentiated more and more, took up less and less space in 
proportion to the whole bulk of the host and formed a germinal mass 
of cells between the outer and inner epitheliallayers. This germinal 
mass formed an epithelium, some of the members of which acted as 
scavengers for the inner and outer layers of the host, with the result 
that fluid accumulated between the two parts of the germinal 
epithelium in connection respectively with the external and internal 
epithélial surfaces of the host, and thus led to the formation of a 
gonoceele, which, by obtaining an external opening, a celomostome, 
gave origin to the ccelom. 
Again, with the longer life of the host, the setting free of the 
gonads no longer necessitating the destruction of the host, and also 
the gonads themselves requiring a longer and longer time to be fed 
up to maturity, the bulk and complexity of the whole organism 
increased and special supporting structures became a necessity. The 
host itself could and did provide these to a certain extent by secre- 
tions from its epithelial elements, but the intermediate supports were 
provided by the system of phagocytic cells utilizing the fluids of the 
body, at first in the shape of plasma-cells able to move from place to 
place, then settling down to form a connective tissue framework, and, 
later on, cartilage and bone. 
So also were gradually evolved the whole of the endothelial 
structures ; the lymph-cells, blood-cells, etc., all having their origin 
from the free cells of the body, which themselves originated in 
the extension of a germinal epithelium. Just as in a bee-hive the 
egg-cells may form the fully developed sexual animal, whether drone 
or queen bee, or the asexual host of workers, so in the body of the 
Metazoa the free cells may form either male or female germ-cells 
spermatozoa, or ova, or a host of workers, scavengers, repairers, food- 
providers, all useful to the community, all showing their common 
origin by their absolute independence of the nervous system. 
Two points of great importance follow from this method of looking 
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