THE PRINCIPLES OF EMBRYOLOGY 48 1 



utilized either as yolk-cells for the providing of pabulum to the egg- 

 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 epithelial layers. 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 

 epithelial surfaces of the host, and thus led to the formation of a 

 gonocoele, which, by obtaining an external opening, a ccelomostome, 

 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 recpiiiring 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 

 plaee, 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 



2 I 



