DISTOMUM CIRRIGERUM. 297 



(12) Embryo escapes from its cercaria-cyst. 



(18) Embryo burrows into the muscles of its host. CEsopha- 

 gus is formed, and excretory exit-duct elongates and becomes 

 tortuous, and tlie cluster of glandular cells around it are 

 developed. The V-shaped excretory bladder becomes globular. 



The interpretation which may be given to these observa- 

 tions will vary according to the preconceptions of the ob- 

 server, and on the definition given to the term '*' germ-layers." 

 A gei"m-layer may be defined as amass or layer of cells which 

 is differentiated from the rest of the body quite early in 

 embryonic life, and out of which certain organs and these 

 alone are produced. From the point of view that ectoderm, 

 endoderm, mesoderm, and fundament of gonad are 

 morphologically homologous parts throughout the animal 

 kingdom it would appear extremely doubtful whether in the 

 present development a true ectoderm or even a true endo- 

 dei'm is formed. The cortex is produced simply by the outer- 

 most undifferentiated cells of the embryo which persist in the 

 adult as the " epidermal " cells. 



If, however, the " germ-layer " is merely a physiological 

 conception of the relative relationships of different layers of 

 the embi-yo there is nothing against calling the cortex-pro- 

 ducing layer ectoderm^ for then the term bears no meaning 

 except that it is the layer of the embryo in contact with the 

 surroun din g medium . 



It should be remembered, however, that the development 

 of the present embryo is obviously a much abbreviated one. 

 This is clearly shown by the curious splitting of the inner 

 sheet of cortex to form the ventral surface, and also by the 

 early appearance of the fundament of the cirrus-sac. But 

 the general tendency of recent enibryological work is certainly 

 in the dii'ection of weakening and not strengthening the con- 

 ception of the morphological significance of germ-layers ; for 

 example, the budding of certain tunicates from the ectoderm 

 alone, the budding of medusa) from the ectoderm of the 

 manubrium (e. g. in Margellium), the apparent formation of 

 a part of the nervous system in echinoderms from cells which 



