266 EMBRYOLOGY OF THE LOWEE VERTEBRATES oh. 



represents the greatly enlarged tubule system of a single segment the 

 ureter would probably have arisen simply by the enlargement of the 

 collecting-tube of that segment. When one studies the facts of 

 development as now known (see p. 258 and especially Fig. 138) the 

 balance of probability appears to be decidedly in favour of the second 

 of these hypotheses representing the method by which the ureter has 

 actually arisen in phylogeny. 



The Gonad. — The great mass of the cells which constitute the 

 body of a Vertebrate or any other of the higher Metazoa are specialized 

 for the performance of particular functions in the ordinary life of the 

 individual, and, correlated with this specialization, such cells have lost 

 the power of giving rise to reproductive cells or gametes. The main mass 

 of the body constituted of such specialized cells is known technically as 

 the soma. At one or more points in the body there remain however 

 patches of cells which have not undergone this specialization for 

 ordinary vital functions and which retain the power of giving rise under 

 favourable circumstances to gametes. The sum total of such cells con- 

 stitute the gonad. The word gonad is commonly used in a loose sense 

 as an equivalent of ovary or testis but it should be borne in mind that 

 each of these organs contains a large proportion of immigrant tissues 

 — connective tissue, blood, nerves and so on — which are strictly 

 speaking part of the soma. 



The problem of greatest general importance attaching to the 

 development of the gonad of Vertebrates is that which concerns the 

 origin of the cells (gonocytes) which constitute it. 



And the interest of this question rests especially on the fact that 

 in certain invertebrates the germ-cells have been traced back to 

 blastomeres specially set apart at early stages of segmentation. 

 All the probabilities seem to indicate that such a process if it occurs 

 in the animal kingdom at all, is of a fundamental character and that 

 indications of the same process may be confidently looked for in 

 other groups. 



The most however that we seem to be justified in asserting to be 

 definitely established for Vertebrates is that genital cells are derived 

 from the mesoderm of the coelomic wall. Apart from the actual 

 facts of observation such development of gonocytes from coelomic 

 lining fits in well with general morphological ideas. It is clear that 

 we must believe that in the. simplest diploblastic ancestor of the 

 Vertebrates the gonocytes were derived from epithelial cells. It is also 

 clear that, on the view that the Coelomata passed through an Actino- 

 zoan-like stage during their evolution, we must regard it as probable 

 that during that stage the gonocytes were situated, as in existing 

 Actinozoa, in the endodermal epithelium lining the pockets between 

 the mesenteries — an epithelium which, on that view, is represented 

 by the endoderm of the enterocoelic pouch of an Amphioxus embryo 

 and by its derivative the coelomic mesoderm of an adult Amphioxus 

 or other Vertebrate. 



In Amphioxus the gonad of the adult shows special peculiarities 



