430 PROFESSOR LANKESTER. 



nephridia form the duets for the testis. Finally, a meta- 

 nephron, with metanephric duct distinct from the Wolffian 

 or mesonephric duct, may develop by a later increase of 

 nephridia posteriorly. This metanephron with niotanephric 

 duct exists in sharks; in abranchiate Vertebrates it becomes 

 the permanent kidney, and its duct the ureter. 



V. Later developihent of the Deron and Enteron. 



In what has preceeded we have given the outline of the 

 cellular foundations of the superficial and deep tissues of the 

 body-wall — of the lining membrane and corpuscles of the 

 lymphatic hsemal spaces and vessels, and of the internal 

 and external tissues of the alimentary tract. 



Oric/i?i of Nerve-tissues. — It only remains to point out 

 briefly, in order to complete this sketch, that the ectoderm 

 having broken up (as we saw above) into epiblast and the 

 musculo-skeletal portion of the mesoblast — or, as we may 

 put it, into neurodermal and myoskeletal moieties — now 

 proceeds further in differentiation. For the neurodermal 

 tissue, which in position is the true representative of the 

 original ectoderm, no%v separates into neural and dermal 

 groups of cells. This does not, however, occur by a general 

 splitting of the neuroderra or epiblast, but by localised 

 differentiation. In all classes of organisms possessing 

 nerve-centres or masses of nerve ganglion-cells these 

 structures have been traced in development to the neuroderm 

 or epiblast. It is probable that primitively the whole ner- 

 vous apparatus is to be traced to epiblast, and that where 

 (as appears very frequently to be the case) masses of nerve- 

 cells and fibres arise deeply by differentiation of cells lying 

 in the mesoblast, such nervous structures are not to be 

 supposed to have taken their origin by a gradual metamor- 

 phosis of musculo-skeletal or of vascular elements, but their 

 present ontogenetic development at points devoid of direct con- 

 nection with epiblast is to be explained as we have explained 

 other shiftings from ancestral connections — namely, by a 

 very early passing over of hereditary nervous molecules from 

 cells destined to form epiblast into cells destined to form 

 mesoblast. 



In all Prostomiata, or, what is same thing, in all Bila- 

 leria or ccelomate Enterozoa, the main tracks occupied 

 by the differentiated nervous tissue have the same position. 

 They appear primarily as paired laterally placed centres 

 within the prostornium. From them radiate fibrous tracts. 

 Their further development consists in elongation, so that 

 they become lateral cords, and the fibrous and spherical 



